channeldb: optimise FetchChannel method

This commit adds a small optimisation to the FetchChannel method.
Instead of iterating over each channel bucket, an identifiable error is
thrown once the wanted channel is found so that the iteration can stop
early.

channeldb/db.go

@@ -661,6 +661,10 @@ func (c *ChannelStateDB) FetchChannel(tx kvdb.RTx, chanPoint wire.OutPoint) (
 	var (
 		targetChan      *OpenChannel
 		targetChanPoint bytes.Buffer
+
+		// errChanFound is used to signal that the channel has been
+		// found so that iteration through the DB buckets can stop.
+		errChanFound = errors.New("channel found")
 	)
 
 	if err := writeOutpoint(&targetChanPoint, &chanPoint); err != nil {
@@ -739,7 +743,7 @@ func (c *ChannelStateDB) FetchChannel(tx kvdb.RTx, chanPoint wire.OutPoint) (
 				targetChan = channel
 				targetChan.Db = c
 
-				return nil
+				return errChanFound
 			})
 		})
 	}
@@ -750,7 +754,7 @@ func (c *ChannelStateDB) FetchChannel(tx kvdb.RTx, chanPoint wire.OutPoint) (
 	} else {
 		err = chanScan(tx)
 	}
-	if err != nil {
+	if err != nil && !errors.Is(err, errChanFound) {
 		return nil, err
 	}