[Enhancement] Parallelization of queue worker

[giranm]

The current implementation of ratelimitqueue assumes a single worker thread which processes events; suitable for PagerDuty's current rate limit (120 per minute per key).

If PagerDuty decides to increase their rate limit per integration key, the event handler should be updated to process more events to meet the new rate limit. This is possible by ensuring queue is processed by a pool of workers in parallel.

Example implementation: https://github.com/JohnPaton/ratelimitqueue/blob/master/examples/example_workers.py