discovery: replace GossipSyncer syncChanUpdates flag with SyncerType

In this commit, we introduce a new type: SyncerType. This type denotes
the type of sync a GossipSyncer is currently under. We only introduce
the two possible entry states, ActiveSync and PassiveSync. An ActiveSync
GossipSyncer will exchange channels with the remote peer and receive new
graph updates from them, while a PassiveSync GossipSyncer will not and
will only response to the remote peer's queries.

This commit does not modify the behavior and is only meant to be a
refactor.

discovery/syncer_test.go

@@ -120,10 +120,9 @@ func newTestSyncer(hID lnwire.ShortChannelID,
 
 	msgChan := make(chan []lnwire.Message, 20)
 	cfg := gossipSyncerCfg{
-		syncChanUpdates: true,
-		channelSeries:   newMockChannelGraphTimeSeries(hID),
-		encodingType:    encodingType,
-		chunkSize:       chunkSize,
+		channelSeries: newMockChannelGraphTimeSeries(hID),
+		encodingType:  encodingType,
+		chunkSize:     chunkSize,
 		sendToPeer: func(msgs ...lnwire.Message) error {
 			msgChan <- msgs
 			return nil