firebase-ios-sdk

Implement safe, synchronous destruction in Firestore (#5620)

Now that the Executor makes strong guarantees that tasks submitted to it will either be completely run or not start at all once destruction starts (#5547), build a Dispose protocol on top of this to allow Firestore to shut itself down leaf first.

Now that Firestore calls the Dispose chain down to the Executor, it's no longer possible for tasks to run while the instance is partially destructed nor can user callbacks execute after the instance has been destroyed.

This is the other half of the iOS changes required to address firebase/quickstart-unity#638.

There are a few follow-on tasks that this PR does not address:

  * Ownership of FirestoreClient and AsyncQueue are still shared though they no longer need to be.
  * The tests are still overly defensive about keeping different test cases isolated from each other. Much of this can be removed now that we're guaranteed to release resources by the time the test case shuts down (at least in GoogleTest; XCTest remains to be seen).