fix: stop poller when input is being shutdown

[yaauie]

:stop_polling
If you throw :stop_polling from the #before_request callback, then the poller will exit normally before making the next long poll request.
-- Aws::SQS::QueuePoller#before_request

Fixes: #33
Closes: #40

[yaauie]

Unfortunately, the existing specs mock too much and this makes no difference to them. But in practice, this should prevent an internal loop of QueuePoller#poll that is receiving zero messages from continuing to poll.

[kares]

tested this locally with AWS and was able to confirm the shutdown stalling no longer ended up in an infinite shutdown loop ... shutdown is stalled only till the next poll kicks in 👍

[JPLachance]

Hello! Do we have an ETA for the merge and release of this fix? It fix issues in our environments. Thank you!