fix(JitsiConference): case for stopping JVB transfer

If for any reason invite for the JVB JingleSession is delayed and
arrives after the P2P connection has been established then
the media transfer needs to be disabled after the offer is accepted.

JitsiConference.js

     try {
         jingleSession.acceptOffer(
             jingleOffer,
-            null /* success */,
+            () => {
+                // If for any reason invite for the JVB session arrived after
+                // the P2P has been established already the media transfer needs
+                // to be turned off here.
+                if (this.isP2PActive() && this.jvbJingleSession) {
+                    this._suspendMediaTransferForJvbConnection();
+                }
+            },
             error => {
                 GlobalOnErrorHandler.callErrorHandler(error);
                 logger.error(
 
     // Stop media transfer over the JVB connection
     if (this.jvbJingleSession) {
-        // FIXME if for whatever reason invite from Jicofo for the JVB
-        // connection arrives, after the P2P has been established
-        // this needs to be called as well.
         this._suspendMediaTransferForJvbConnection();
     }
 

modules/xmpp/JingleSessionPC.js

      */
     setMediaTransferActive(active) {
         const workFunction = finishedCallback => {
+
+            // Because the value is modified on the queue it's impossible to
+            // check it's final value reliably prior to submitting the task.
+            // The rule here is that the last submitted state counts.
+            // Check the value here to avoid unnecessary renegotiation cycle.
+            if (this.mediaTransferActive === active) {
+                finishedCallback();
+
+                return;
+            }
             this.mediaTransferActive = active;
             if (this.peerconnection) {
                 this.peerconnection.setMediaTransferActive(