Optimize EventSource Workers

[ipublic]

EventSource spawns queue workers to process messages. Add following optimizations:

1. Auto-terminate workers after a period of inactivity.
   1. 1. Set default time period by ENV variable: EVENT_SOURCE_WORKER_TIMEOUT
   2. Enable override on a consumer basis
2. Set the max number of workers per queue.
   1. Default set by ENV variable.: EVENT_SOURCE_WORKER_MAX
   2. Enable override on a consumer basis

Aha! Link: https://ideacrew.aha.io/features/EVTSRC-7

[TreyE]

Also might cross with worker pool implementation (if that is the route selected) under: #111.

Keep in mind I believe we have two different backing behaviours/work spawning implementations under event source:

1. HTTP protocol worker
2. Whatever thread/pool worker is provided natively by the Bunny gem for it's AMQP subscribers

There is also #78 (which I'm planing to re-write and possibly discard - I don't care for some of the defaults I've chosen and how they may clash with the default expected/current deployment behaviour of ES).