firebase-ios-sdk

Fixes for issues discovered in the Unity desktop build (#4795)

* Make Filesystem's destructor virtual

* Avoid capturing a strong reference to FirestoreClient in GC tasks.

This prevents this task from keeping the FirestoreClient alive even
after the api::Firestore instance has been destroyed.

* Ensure that FirestoreClient is terminated

In the public C++ API, it's possible to delete the owning FirebaseApp
which triggers the destruction of the api::Firestore object even though
we haven't called Terminate.

This makes it such that api::Firestore calls a blocking Terminate call
on FirestoreClient in its destructor.

Additionally, change AsyncQueue to always run the operation enqueued on
shutdown, even if already shutdown. Make FirestoreClient capable of
short circuiting the termination work for itself. This makes terminate
idempotent in a more staightforward way.

* Clean up excess logging

* Remove defintion of weak_from_this

* Review feedback

* Make AsyncQueue extend std::enable_shared_from_this

Also hide the AsyncQueue's public constructor and force all users to
manage their reference to it via std::shared_ptr.

* Handle Executor shutdown from within its own pool