lnutil: change ReceivedMPPStatus.htlcs to frozenset, i.e. immutable

diff --git a/electrum/lnpeer.py b/electrum/lnpeer.py
index 17b836d..ae525ab 100644
--- a/electrum/lnpeer.py
+++ b/electrum/lnpeer.py
@@ -2200,7 +2200,7 @@ class Peer(Logger, EventListener):
         # get payment hash of any htlc in the set (they are all the same)
         payment_hash = htlc_set.get_payment_hash()
         assert payment_hash is not None, htlc_set
-        assert payment_hash not in self.lnworker.dont_settle_htlcs
+        assert payment_hash.hex() not in self.lnworker.dont_settle_htlcs
         self.lnworker.dont_expire_htlcs.pop(payment_hash.hex(), None)  # htlcs wont get expired anymore
         for mpp_htlc in list(htlc_set.htlcs):
             htlc_id = mpp_htlc.htlc.htlc_id
@@ -2214,10 +2214,12 @@ class Peer(Logger, EventListener):
             if chan.hm.was_htlc_preimage_released(htlc_id=htlc_id, htlc_proposer=REMOTE):
                 # this check is intended to gracefully handle stale htlcs in the set, e.g. after a crash
                 self.logger.debug(f"{mpp_htlc=} was already settled before, dropping it.")
-                htlc_set.htlcs.remove(mpp_htlc)
+                htlc_set = htlc_set._replace(htlcs=htlc_set.htlcs - {mpp_htlc})
+                self.lnworker.received_mpp_htlcs[payment_key] = htlc_set
                 continue
             self._fulfill_htlc(chan, htlc_id, preimage)
-            htlc_set.htlcs.remove(mpp_htlc)
+            htlc_set = htlc_set._replace(htlcs=htlc_set.htlcs - {mpp_htlc})
+            self.lnworker.received_mpp_htlcs[payment_key] = htlc_set
             # reset just-in-time opening fee of channel
             chan.jit_opening_fee = None
 
@@ -2255,7 +2257,8 @@ class Peer(Logger, EventListener):
             if chan.hm.was_htlc_failed(htlc_id=htlc_id, htlc_proposer=REMOTE):
                 # this check is intended to gracefully handle stale htlcs in the set, e.g. after a crash
                 self.logger.debug(f"{mpp_htlc=} was already failed before, dropping it.")
-                htlc_set.htlcs.remove(mpp_htlc)
+                htlc_set = htlc_set._replace(htlcs=htlc_set.htlcs - {mpp_htlc})
+                self.lnworker.received_mpp_htlcs[payment_key] = htlc_set
                 continue
             onion_packet = self._parse_onion_packet(mpp_htlc.unprocessed_onion)
             processed_onion_packet = self._process_incoming_onion_packet(
@@ -2286,7 +2289,8 @@ class Peer(Logger, EventListener):
                 htlc_id=htlc_id,
                 error_bytes=error_bytes,
             )
-            htlc_set.htlcs.remove(mpp_htlc)
+            htlc_set = htlc_set._replace(htlcs=htlc_set.htlcs - {mpp_htlc})
+            self.lnworker.received_mpp_htlcs[payment_key] = htlc_set
 
     def fail_htlc(self, *, chan: Channel, htlc_id: int, error_bytes: bytes):
         self.logger.info(f"fail_htlc. chan {chan.short_channel_id}. htlc_id {htlc_id}.")
@@ -3148,11 +3152,12 @@ class Peer(Logger, EventListener):
             if not parent:
                 parent = ReceivedMPPStatus(
                     resolution=RecvMPPResolution.WAITING,
-                    htlcs=set(),
+                    htlcs=frozenset(),
                 )
-                self.lnworker.received_mpp_htlcs[mpp_set.parent_set_key] = parent
-            parent.htlcs.update(mpp_set.htlcs)
-            mpp_set.htlcs.clear()
+            self.lnworker.received_mpp_htlcs[mpp_set.parent_set_key] = parent._replace(
+                htlcs=parent.htlcs | mpp_set.htlcs
+            )
+            self.lnworker.received_mpp_htlcs[payment_key] = mpp_set._replace(htlcs=frozenset())
             return None, None, None  # this set will get deleted as there are no htlcs in it anymore
 
         assert not mpp_set.parent_set_key
diff --git a/electrum/lnutil.py b/electrum/lnutil.py
index ff33f39..9512721 100644
--- a/electrum/lnutil.py
+++ b/electrum/lnutil.py
@@ -1980,7 +1980,7 @@ class ReceivedMPPHtlc(NamedTuple):
 
 class ReceivedMPPStatus(NamedTuple):
     resolution: RecvMPPResolution
-    htlcs: set[ReceivedMPPHtlc]
+    htlcs: frozenset[ReceivedMPPHtlc]
     # parent_set_key is needed as trampoline allows MPP to be nested, the parent_set_key is the
     # payment key of the final mpp set (derived from inner trampoline onion payment secret)
     # to which the separate trampoline sets htlcs get added once they are complete.
@@ -2005,7 +2005,7 @@ class ReceivedMPPStatus(NamedTuple):
     @stored_in('received_mpp_htlcs', tuple)
     def from_tuple(resolution, htlc_list, parent_set_key=None) -> 'ReceivedMPPStatus':
         assert isinstance(resolution, int)
-        htlc_set = set(ReceivedMPPHtlc.from_tuple(*htlc_data) for htlc_data in htlc_list)
+        htlc_set = frozenset(ReceivedMPPHtlc.from_tuple(*htlc_data) for htlc_data in htlc_list)
         return ReceivedMPPStatus(
             resolution=RecvMPPResolution(resolution),
             htlcs=htlc_set,
diff --git a/electrum/lnworker.py b/electrum/lnworker.py
index a255c09..453de6d 100644
--- a/electrum/lnworker.py
+++ b/electrum/lnworker.py
@@ -2807,7 +2807,7 @@ class LNWallet(Logger):
             self.logger.debug(f"creating new mpp set for {payment_key=}")
             mpp_status = ReceivedMPPStatus(
                 resolution=RecvMPPResolution.WAITING,
-                htlcs=set(),
+                htlcs=frozenset(),
             )
 
         if mpp_status.resolution > RecvMPPResolution.WAITING:
@@ -2827,8 +2827,9 @@ class LNWallet(Logger):
         )
         assert new_htlc not in mpp_status.htlcs, "each htlc should make it here only once?"
         assert isinstance(unprocessed_onion_packet, str)
-        mpp_status.htlcs.add(new_htlc)  # side-effecting htlc_set
-        self.received_mpp_htlcs[payment_key] = mpp_status
+        new_htlcs = set(mpp_status.htlcs)
+        new_htlcs.add(new_htlc)
+        self.received_mpp_htlcs[payment_key] = mpp_status._replace(htlcs=frozenset(new_htlcs))
 
     def set_mpp_resolution(self, payment_key: str, new_resolution: RecvMPPResolution) -> ReceivedMPPStatus:
         mpp_status = self.received_mpp_htlcs[payment_key]
@@ -2910,10 +2911,14 @@ class LNWallet(Logger):
         assert chan._state == ChannelState.REDEEMED
         for payment_key_hex, mpp_status in list(self.received_mpp_htlcs.items()):
             htlcs_to_remove = [htlc for htlc in mpp_status.htlcs if htlc.channel_id == chan.channel_id]
+            new_htlcs = set(mpp_status.htlcs)
             for stale_mpp_htlc in htlcs_to_remove:
                 assert mpp_status.resolution != RecvMPPResolution.WAITING
                 self.logger.info(f'maybe_cleanup_mpp: removing htlc of MPP {payment_key_hex}')
-                mpp_status.htlcs.remove(stale_mpp_htlc)  # side-effecting htlc_set
+                new_htlcs.remove(stale_mpp_htlc)
+            if htlcs_to_remove:
+                mpp_status = mpp_status._replace(htlcs=frozenset(new_htlcs))
+                self.received_mpp_htlcs[payment_key_hex] = mpp_status  # save changes to db
             if len(mpp_status.htlcs) == 0:
                 self.logger.info(f'maybe_cleanup_mpp: removing mpp {payment_key_hex}')
                 del self.received_mpp_htlcs[payment_key_hex]
diff --git a/electrum/util.py b/electrum/util.py
index 8e5a391..2ef8e2d 100644
--- a/electrum/util.py
+++ b/electrum/util.py
@@ -331,7 +331,7 @@ class MyEncoder(json.JSONEncoder):
         if isinstance(obj, datetime):
             # note: if there is a timezone specified, this will include the offset
             return obj.isoformat(' ', timespec="minutes")
-        if isinstance(obj, set):
+        if isinstance(obj, (set, frozenset)):
             return list(obj)
         if isinstance(obj, bytes): # for nametuples in lnchannel
             return obj.hex()