paymentsdb: move serialization methods to kv_store file

diff --git a/payments/db/kv_store.go b/payments/db/kv_store.go
index afc7dec..343f77e 100644
--- a/payments/db/kv_store.go
+++ b/payments/db/kv_store.go
@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 	"sort"
 	"sync"
 	"time"
@@ -2107,3 +2108,91 @@ func DeserializeRoute(r io.Reader) (route.Route, error) {
 
 	return rt, nil
 }
+
+// serializeHTLCSettleInfo serializes the details of a settled htlc.
+func serializeHTLCSettleInfo(w io.Writer, s *HTLCSettleInfo) error {
+	if _, err := w.Write(s.Preimage[:]); err != nil {
+		return err
+	}
+
+	if err := serializeTime(w, s.SettleTime); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// deserializeHTLCSettleInfo deserializes the details of a settled htlc.
+func deserializeHTLCSettleInfo(r io.Reader) (*HTLCSettleInfo, error) {
+	s := &HTLCSettleInfo{}
+	if _, err := io.ReadFull(r, s.Preimage[:]); err != nil {
+		return nil, err
+	}
+
+	var err error
+	s.SettleTime, err = deserializeTime(r)
+	if err != nil {
+		return nil, err
+	}
+
+	return s, nil
+}
+
+// serializeHTLCFailInfo serializes the details of a failed htlc including the
+// wire failure.
+func serializeHTLCFailInfo(w io.Writer, f *HTLCFailInfo) error {
+	if err := serializeTime(w, f.FailTime); err != nil {
+		return err
+	}
+
+	// Write failure. If there is no failure message, write an empty
+	// byte slice.
+	var messageBytes bytes.Buffer
+	if f.Message != nil {
+		err := lnwire.EncodeFailureMessage(&messageBytes, f.Message, 0)
+		if err != nil {
+			return err
+		}
+	}
+	if err := wire.WriteVarBytes(w, 0, messageBytes.Bytes()); err != nil {
+		return err
+	}
+
+	return WriteElements(w, byte(f.Reason), f.FailureSourceIndex)
+}
+
+// deserializeHTLCFailInfo deserializes the details of a failed htlc including
+// the wire failure.
+func deserializeHTLCFailInfo(r io.Reader) (*HTLCFailInfo, error) {
+	f := &HTLCFailInfo{}
+	var err error
+	f.FailTime, err = deserializeTime(r)
+	if err != nil {
+		return nil, err
+	}
+
+	// Read failure.
+	failureBytes, err := wire.ReadVarBytes(
+		r, 0, math.MaxUint16, "failure",
+	)
+	if err != nil {
+		return nil, err
+	}
+	if len(failureBytes) > 0 {
+		f.Message, err = lnwire.DecodeFailureMessage(
+			bytes.NewReader(failureBytes), 0,
+		)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	var reason byte
+	err = ReadElements(r, &reason, &f.FailureSourceIndex)
+	if err != nil {
+		return nil, err
+	}
+	f.Reason = HTLCFailReason(reason)
+
+	return f, nil
+}
diff --git a/payments/db/payment.go b/payments/db/payment.go
index 72338a4..2c5931a 100644
--- a/payments/db/payment.go
+++ b/payments/db/payment.go
@@ -4,12 +4,9 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
-	"io"
-	"math"
 	"time"
 
 	"github.com/btcsuite/btcd/btcec/v2"
-	"github.com/btcsuite/btcd/wire"
 	"github.com/davecgh/go-spew/spew"
 	sphinx "github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/lntypes"
@@ -650,94 +647,6 @@ func (m *MPPayment) AllowMoreAttempts() (bool, error) {
 	return true, nil
 }
 
-// serializeHTLCSettleInfo serializes the details of a settled htlc.
-func serializeHTLCSettleInfo(w io.Writer, s *HTLCSettleInfo) error {
-	if _, err := w.Write(s.Preimage[:]); err != nil {
-		return err
-	}
-
-	if err := serializeTime(w, s.SettleTime); err != nil {
-		return err
-	}
-
-	return nil
-}
-
-// deserializeHTLCSettleInfo deserializes the details of a settled htlc.
-func deserializeHTLCSettleInfo(r io.Reader) (*HTLCSettleInfo, error) {
-	s := &HTLCSettleInfo{}
-	if _, err := io.ReadFull(r, s.Preimage[:]); err != nil {
-		return nil, err
-	}
-
-	var err error
-	s.SettleTime, err = deserializeTime(r)
-	if err != nil {
-		return nil, err
-	}
-
-	return s, nil
-}
-
-// serializeHTLCFailInfo serializes the details of a failed htlc including the
-// wire failure.
-func serializeHTLCFailInfo(w io.Writer, f *HTLCFailInfo) error {
-	if err := serializeTime(w, f.FailTime); err != nil {
-		return err
-	}
-
-	// Write failure. If there is no failure message, write an empty
-	// byte slice.
-	var messageBytes bytes.Buffer
-	if f.Message != nil {
-		err := lnwire.EncodeFailureMessage(&messageBytes, f.Message, 0)
-		if err != nil {
-			return err
-		}
-	}
-	if err := wire.WriteVarBytes(w, 0, messageBytes.Bytes()); err != nil {
-		return err
-	}
-
-	return WriteElements(w, byte(f.Reason), f.FailureSourceIndex)
-}
-
-// deserializeHTLCFailInfo deserializes the details of a failed htlc including
-// the wire failure.
-func deserializeHTLCFailInfo(r io.Reader) (*HTLCFailInfo, error) {
-	f := &HTLCFailInfo{}
-	var err error
-	f.FailTime, err = deserializeTime(r)
-	if err != nil {
-		return nil, err
-	}
-
-	// Read failure.
-	failureBytes, err := wire.ReadVarBytes(
-		r, 0, math.MaxUint16, "failure",
-	)
-	if err != nil {
-		return nil, err
-	}
-	if len(failureBytes) > 0 {
-		f.Message, err = lnwire.DecodeFailureMessage(
-			bytes.NewReader(failureBytes), 0,
-		)
-		if err != nil {
-			return nil, err
-		}
-	}
-
-	var reason byte
-	err = ReadElements(r, &reason, &f.FailureSourceIndex)
-	if err != nil {
-		return nil, err
-	}
-	f.Reason = HTLCFailReason(reason)
-
-	return f, nil
-}
-
 // generateSphinxPacket generates then encodes a sphinx packet which encodes
 // the onion route specified by the passed layer 3 route. The blob returned
 // from this function can immediately be included within an HTLC add packet to