…ion limits

* Add new userCallbackExecutor which is used to ensure user callbacks aren't able to block any gax thread. Currently this is creating a cached thread pool per writer, we should likely treat this thread pool as a singleton and pass it from the builder to leverage the reuse of the threads even past the lifetime of the writer.
* Update instances of setting a SettableFuture to only set after the state operation (add/remove) has completed to ensure and downstream future will only start after state update.

…firestore into bc/bulk-error-with-test

… wasting logic