Reuse transceivers (#24)

* protocol 4

* default to v4

* mediaStreamTrack.enabled

* track start

* private constructor

* remote audio/video tracks

* log sdp

* organize imports

lib/src/track/track.dart

@@ -76,6 +76,8 @@ abstract class Track extends DisposableChangeNotifier
       return false;
     }
 
+    logger.fine('Track.start()');
+
     _active = true;
     return true;
   }
@@ -88,13 +90,29 @@ abstract class Track extends DisposableChangeNotifier
       return false;
     }
 
-    try {
-      await mediaStreamTrack.stop();
-    } catch (_) {
-      logger.warning('[$objectId] rtc.mediaStreamTrack.stop() did throw ${_}');
-    }
+    logger.fine('Track.stop()');
 
     _active = false;
     return true;
   }
+
+  Future<void> enable() async {
+    logger.fine('Track.enable()');
+    try {
+      mediaStreamTrack.enabled = true;
+    } catch (_) {
+      logger.warning(
+          '[$objectId] set rtc.mediaStreamTrack.enabled did throw ${_}');
+    }
+  }
+
+  Future<void> disable() async {
+    logger.fine('Track.disable()');
+    try {
+      mediaStreamTrack.enabled = false;
+    } catch (_) {
+      logger.warning(
+          '[$objectId] set rtc.mediaStreamTrack.enabled did throw ${_}');
+    }
+  }
 }