discovery: rate limiting sending msgs per peer

We now add another layer of rate limiting before sending the messages
inside `GossipSyncer`.

discovery/syncer_atomic_test.go

@@ -26,9 +26,14 @@ func TestGossipSyncerSingleBacklogSend(t *testing.T) {
 	// Create a blocking sendToPeerSync function. We'll use this to simulate
 	// sending a large backlog.
 	blockingSendChan := make(chan struct{})
-	sendToPeerSync := func(_ context.Context,
+	mockSendMsg := func(_ context.Context, sync bool,
 		msgs ...lnwire.Message) error {
 
+		// Sync is only true when calling `sendToPeerSync`.
+		if !sync {
+			return nil
+		}
+
 		// Track that we're in a send goroutine.
 		count := activeGoroutines.Add(1)
 		totalGoroutinesLaunched.Add(1)
@@ -55,8 +60,8 @@ func TestGossipSyncerSingleBacklogSend(t *testing.T) {
 		defaultChunkSize, true, true, true,
 	)
 
-	// Override the sendToPeerSync to use our blocking version.
-	syncer.cfg.sendToPeerSync = sendToPeerSync
+	// Override the sendMsg to use our blocking version.
+	syncer.cfg.sendMsg = mockSendMsg
 	syncer.cfg.ignoreHistoricalFilters = false
 
 	syncer.Start()