Session migration (#72)

* impl

* retry logic

* simulate

* pass checks

* adjust log level

* retry condition

* fix simulate scenario params

* format

* use `RTCPeerConnectionState` instead of `RTCIceConnectionState`

* adjust retry

* additional check for completer

* more logging

* fix id

* pub

* change package

* doc update

* retry tests

* format

* apply suggestions

lib/src/managers/event.dart

@@ -79,7 +79,7 @@ abstract class EventsListenable<T> extends Disposable {
   Future<void> cancelAll() async {
     if (_listeners.isNotEmpty) {
       // Stop listening to all events
-      logger.fine('${objectId} cancelling ${_listeners.length} listeners(s)');
+      logger.finer('${objectId} cancelling ${_listeners.length} listeners(s)');
       for (final listener in _listeners) {
         await listener.cancel();
       }
@@ -135,7 +135,11 @@ abstract class EventsListenable<T> extends Disposable {
     final completer = Completer<E>();
 
     final _cancelFunc = on<E>(
-      (event) => completer.complete(event),
+      (event) {
+        if (!completer.isCompleted) {
+          completer.complete(event);
+        }
+      },
       filter: filter,
     );