Reuse transceivers (#24)

* protocol 4

* default to v4

* mediaStreamTrack.enabled

* track start

* private constructor

* remote audio/video tracks

* log sdp

* organize imports

lib/src/signal_client.dart

@@ -24,7 +24,7 @@ class SignalClient extends Disposable with EventsEmittable<SignalEvent> {
   LiveKitWebSocket? _ws;
 
   SignalClient({
-    this.protocol = ProtocolVersion.protocol3,
+    this.protocol = ProtocolVersion.protocol4,
   }) {
     events.listen((event) {
       logger.fine('[SignalEvent] $event');