channeldb/graphdb: move outpoint ser/deser funcs to graphdb

We have the same helpers for writing and reading a wire.Outpoint type
defined separately in a couple places. We will want to use these from
the graph db package soon though so instead of defining them again
there, this commit unifies things and creates a single exported set of
helpers. The next commit will make use of these.

channeldb/db.go

@@ -30,6 +30,7 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb/migration33"
 	"github.com/lightningnetwork/lnd/channeldb/migration_01_to_11"
 	"github.com/lightningnetwork/lnd/clock"
+	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/invoices"
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -646,7 +647,9 @@ func (c *ChannelStateDB) fetchNodeChannels(chainBucket kvdb.RBucket) (
 		chanBucket := chainBucket.NestedReadBucket(chanPoint)
 
 		var outPoint wire.OutPoint
-		err := readOutpoint(bytes.NewReader(chanPoint), &outPoint)
+		err := graphdb.ReadOutpoint(
+			bytes.NewReader(chanPoint), &outPoint,
+		)
 		if err != nil {
 			return err
 		}
@@ -675,7 +678,8 @@ func (c *ChannelStateDB) FetchChannel(tx kvdb.RTx, chanPoint wire.OutPoint) (
 	*OpenChannel, error) {
 
 	var targetChanPoint bytes.Buffer
-	if err := writeOutpoint(&targetChanPoint, &chanPoint); err != nil {
+	err := graphdb.WriteOutpoint(&targetChanPoint, &chanPoint)
+	if err != nil {
 		return nil, err
 	}
 
@@ -709,7 +713,9 @@ func (c *ChannelStateDB) FetchChannelByID(tx kvdb.RTx, id lnwire.ChannelID) (
 		)
 		err := chainBkt.ForEach(func(k, _ []byte) error {
 			var outPoint wire.OutPoint
-			err := readOutpoint(bytes.NewReader(k), &outPoint)
+			err := graphdb.ReadOutpoint(
+				bytes.NewReader(k), &outPoint,
+			)
 			if err != nil {
 				return err
 			}
@@ -1089,7 +1095,7 @@ func (c *ChannelStateDB) FetchClosedChannel(chanID *wire.OutPoint) (
 
 		var b bytes.Buffer
 		var err error
-		if err = writeOutpoint(&b, chanID); err != nil {
+		if err = graphdb.WriteOutpoint(&b, chanID); err != nil {
 			return err
 		}
 
@@ -1131,7 +1137,9 @@ func (c *ChannelStateDB) FetchClosedChannelForID(cid lnwire.ChannelID) (
 		// We scan over all possible candidates for this channel ID.
 		for ; op != nil && bytes.Compare(cid[:30], op[:30]) <= 0; op, c = cursor.Next() {
 			var outPoint wire.OutPoint
-			err := readOutpoint(bytes.NewReader(op), &outPoint)
+			err := graphdb.ReadOutpoint(
+				bytes.NewReader(op), &outPoint,
+			)
 			if err != nil {
 				return err
 			}
@@ -1173,7 +1181,7 @@ func (c *ChannelStateDB) MarkChanFullyClosed(chanPoint *wire.OutPoint) error {
 	)
 	err := kvdb.Update(c.backend, func(tx kvdb.RwTx) error {
 		var b bytes.Buffer
-		if err := writeOutpoint(&b, chanPoint); err != nil {
+		if err := graphdb.WriteOutpoint(&b, chanPoint); err != nil {
 			return err
 		}
 
@@ -1693,7 +1701,7 @@ func fetchHistoricalChanBucket(tx kvdb.RTx,
 	// With the bucket for the node and chain fetched, we can now go down
 	// another level, for the channel itself.
 	var chanPointBuf bytes.Buffer
-	if err := writeOutpoint(&chanPointBuf, outPoint); err != nil {
+	if err := graphdb.WriteOutpoint(&chanPointBuf, outPoint); err != nil {
 		return nil, err
 	}
 	chanBucket := historicalChanBucket.NestedReadBucket(