fix(JitsiConference,RTC): add onCallEnded to the RTC service

JitsiConference.js

     if (this.statistics) {
         this.statistics.stopRemoteStats();
     }
+    // Let the RTC service do any cleanups
+    this.rtc.onCallEnded();
     // PeerConnection has been closed which means that SSRCs stored in
     // JitsiLocalTrack will not match those assigned by the old PeerConnection
     // and SSRC replacement logic will not work as expected.

modules/RTC/RTC.js

             this.eventEmitter);
 };
 
+/**
+ * Should be called when current media session ends and after the PeerConnection
+ * has been closed using PeerConnection.close() method.
+ */
+RTC.prototype.onCallEnded = function() {
+    if (this.dataChannels) {
+        // DataChannels are not explicitly closed as the PeerConnection
+        // is closed on call ended which triggers data channel onclose events.
+        // The reference is cleared to disable any logic related to the data
+        // channels.
+        this.dataChannels = null;
+    }
+};
+
 /**
  * Elects the participant with the given id to be the pinned participant in
  * order to always receive video for this participant (even when last n is