Don't use BackoffThrottler in multithread contexts

[mjameswh]

What was changed

• Avoid using BackoffThrottler instances in multi-thread contexts (Poller)
• Merged AsyncBackoffThrotter into BackoffThrotter; BackoffThrottler now has a throttleAsync() method

Why?

• BackoffThrottler was not designed to provide an external thread safe semantic; consequently, a race condition between several threads could result in skipping throttling or in throttling more than expected.
• These changes will make it much easier to implement longer backoff time on RESOURCE_EXHAUSTION failures.