discovery: introduce rate limiter to GossipSyncer

discovery/sync_manager.go

@@ -40,6 +40,11 @@ const (
 	// they'll be refilled at this rate.
 	DefaultMsgBytesPerSecond = 1000 * 1_024
 
+	// DefaultPeerMsgBytesPerSecond is the max bytes/s we'll permit for
+	// outgoing messages for a single peer. Once tokens (bytes) have been
+	// taken from the bucket, they'll be refilled at this rate.
+	DefaultPeerMsgBytesPerSecond = 50 * 1_024
+
 	// assumedMsgSize is the assumed size of a message if we can't compute
 	// its serialized size. This comes out to 1 KB.
 	assumedMsgSize = 1_024