This solves #69.  This seems to be the most performant way to enable the ability to get a queue size.

This expands the constructor amount quite a lot, but it does offer the new feature of being able to query for the queue size for a given key.

Based on my benchmarks, this change has minimal (if any) performance penalties.
Merge pull request #73 from jentfoo/TaskExecutorDistributor-getTaskQueueCount-switchable_accuracy

Add ability to query task queue length for a given thread key (third pull request)