fix: Race condition where future callbacks invoked before client is in paused state

[mukund-ananthu]

For non-retriable errors returned from the backend, the client library currently sets future.set_exception() for all outstanding futures:

python-pubsub/google/cloud/pubsub_v1/publisher/_batch/thread.py

Lines 285 to 286 in 8c7e2a9

	for future in self._futures:
	future.set_exception(exc)

before it tries to change its state to paused by calling _batch_done_callback():

python-pubsub/google/cloud/pubsub_v1/publisher/_batch/thread.py

Lines 289 to 291 in 8c7e2a9

	if self._batch_done_callback is not None:
	# Failed to publish batch.
	self._batch_done_callback(batch_transport_succeeded)

which transitions the state of the client library to a paused state:

python-pubsub/google/cloud/pubsub_v1/publisher/_sequencer/ordered_sequencer.py

Lines 204 to 206 in ff229a5

	else:
	# Unrecoverable error detected
	self._pause()

This results in race conditions where the callback associated with the future is invoked before the client enters paused state. This causes issues if the callback attempts to do a resume_publish() before the client is in the paused state.

This fix ensures that the client is in paused state before future callbacks are invoked as a consequence of future.set_exception

Fixes #1144 🦕