Clear iOS platform instances synchronously

[ben-xD]

I'm moving this code from running in the background, to instead run synchronously.

This fixes a race condition bug where dispose is first called from the Dart side to clear all the instances of realtime and rest. At this point, the realtime/rest clients are not yet cleared, and the work is queued. The actual implementation of dispose might only happen afterwards, because of dispatch_async. By that time, a user might have created a new realtime instance. Then, this dispose method finally gets called, and their client is deleted, after they create it.

This was actually discovered because I removed the button create realtime, and created the realtime client on launch instead. User applications would likely create a realtime instance either on application launch or user log in too.

Having said that, adding buttons for closing realtime and re-creating it can be useful to debug issues.

[ben-xD]

Here is an explanation of the existing buggy code:

When a users application (or example app) launches for the first time, or a hot restart happens (Flutter app restarts without restarting iOS host application), the Dart side calls a method: registerWithCompletionHandler. This calls AblyFlutter disposeWithCompletionHandler.

Bug: The registerWithCompletionHandler method is first called when the app launches, but the clearing does not happen immediately. Then, the user might programmatically create a realtime at app launch too. This saves a realtime client in AblyFlutter.m's _realtimeInstances. However, finally, the following code is executed in the background:

    for (ARTRealtime *const r in _realtimeInstances.allValues) {
        [r close];
    }
    [_realtimeInstances removeAllObjects];
    [_restInstances removeAllObjects];

deleting the users realtime client, when they didn't intend for it to be deleted.