Reuse transceivers in subscribe path (#51)

* Reuse transceivers in subscribe path

LK-22 (https://linear.app/livekit/issue/LK-22/feature-pooled-transceivers-to-avoid-re-negotiation)

Do not stop remote tracks. That would change `readyState` of track
to `ended` and the corresponding transceiver cannot be reused.
Instead use the `enabled` attribute of track to enable/disable
remote tracks.

Bumping up the protocol version that informs the server that
client is capable of reusing transceivers.

Testing:
--------
Connect from Chrome. For remote side, connect/disconnect
from Firefox several times and ensure the following
- SDP does not grow (it has 4 sections as reported by
chrome://webrtc-internals (version, data, audio, video) each
time the remote side connects.
- Media flows from remote side on every connection.

Oddities:
---------
- Chrome callback `ontrack` sends a track. When the remote side
reconnects and the same transceiver is used, the callback does
not contain the new track id. It retains what was returned in the
first callback. Don't think it is an issue as we do not use that
track id anywhere.

* Apply suggestions from code review

Co-authored-by: David Zhao <[email protected]>

Co-authored-by: David Zhao <[email protected]>

src/room/participant/RemoteParticipant.ts

@@ -104,6 +104,7 @@ export default class RemoteParticipant extends Participant {
     } else {
       track = new RemoteAudioTrack(mediaTrack, sid, receiver);
     }
+    track.start();
 
     publication.setTrack(track);
     // set track name etc

src/room/track/RemoteAudioTrack.ts

@@ -22,4 +22,14 @@ export default class RemoteAudioTrack extends Track {
       this.emit(muted ? TrackEvent.Muted : TrackEvent.Unmuted, this);
     }
   }
+
+  start() {
+    // use `enabled` of track to enable re-use of transceiver
+    super.enable();
+  }
+
+  stop() {
+    // use `enabled` of track to enable re-use of transceiver
+    super.disable();
+  }
 }

src/room/track/RemoteVideoTrack.ts

@@ -37,7 +37,13 @@ export default class RemoteVideoTrack extends Track {
     });
   }
 
+  start() {
+    // use `enabled` of track to enable re-use of transceiver
+    super.enable();
+  }
+
   stop() {
-    super.stop();
+    // use `enabled` of track to enable re-use of transceiver
+    super.disable();
   }
 }

src/room/track/Track.ts

@@ -110,6 +110,14 @@ export class Track extends EventEmitter {
     this.mediaStreamTrack.stop();
   }
 
+  protected enable() {
+    this.mediaStreamTrack.enabled = true;
+  }
+
+  protected disable() {
+    this.mediaStreamTrack.enabled = false;
+  }
+
   private recycleElement(element: HTMLMediaElement) {
     if (element instanceof HTMLAudioElement) {
     // we only need to re-use a single element

src/version.ts

@@ -1,2 +1,2 @@
 export const version = '0.12.2';
-export const protocolVersion = 3;
+export const protocolVersion = 4;