fix(firestore): Prevent iOS crash caused by concurrency issue

[TomCarey]

Close #771

Wraps the listenerRegistrationMap in an actor to manage the reading and writing in a thread-safe way. It is necessary because addCollectionSnapshotListener and addCollectionGroupSnapshotListener dispatch a Task in order to build the constraints but then immediately write to the listenerRegistrationMap within that Task.

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[TomCarey]

@robingenz I think there is an error introduced in your code suggestion. The dispatch queue of the capacitor bridge is internal so we can't dispatch work onto that queue. The event listeners of CAPPlugin are stored in a dictionary on that bridge queue. By calling super in the completion block we are running removeEventListeners in a different thread so there is a race condition where we might unsafely manipulate that dictionary. I'm not sure there is a simple solution here. It seems like it would be nice for capacitor to expose that queue. In theory though it would probably be "safer" to run the code in the way that I had it before where we didn't call super.removeAllListeners but instead extract that behaviour like we had it before.

[robingenz]

@TomCarey You are right. Please revert this change. We'll try your approach first.

[TomCarey]

Thanks @robingenz I've reverted that now.

[robingenz]

Thank you!

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commit: 48a9593