Add BoundedQueue to AsyncDispatcher

I recently came across a scenario where an MR ApplicationMaster was failing with an OOM exception. It had many thousands of Mappers and thousands of Reducers. It was noted that in the logging that the event-queue of AsyncDispatcher had a very large number of item in it and was seemingly never decreasing.

I started looking at the code and thought it could use some clean up, simplification, and the ability to specify a bounded queue so that any incoming events are throttled until they can be processed. This will protect the ApplicationMaster from a flood of events.

Logging Message:
Size of event-queue is xxx