ensure we use retry classes correctly for {unary,streaming}×{sync,async} methods

[vchudnov-g]

As per googleapis/python-api-core#528, GAPIC libraries currently have a race condition with unary async retries, which should be fixed by googleapis/python-api-core#495. And the GAPICs also do not handle the new streaming retry classes introduced in googleapis/python-api-core#495.

Once a version python-api-core with that merged PR gets released, we need to make sure that we use retries appropriately for streaming methods, in all transports. Also see this comment in that PR.