Defend against users making API calls during deinit (#863)

Track operation in progress FSTDispatchQueue

... and allow dispatchAsync onto the worker queue even from the same
queue if an operation is not in progress. Fixes #753.

This fixes a corner case that happens when users pass us an object and
we retain that object in a block submitted to the worker queue for
processing. Users are not obligated to retain that object themselves.
This can lead to the object's destructor (or dealloc or deinit) being
called on the Firestore worker thread. If that destructor makes
Firestore API calls (most likely ListenerRegistration.remove but any are
possible) this will trigger the assertion.

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