Allow idle thread reuse

XNIO thread pool is configured to always have core size == max size, which causes thread count to always increase to maximum and idle threads are never allowed to die.

This makes it impossible to configure the system so that there would be sufficient, small number of threads to handle requests, but have a big number of reserve threads. Huge number of threads is a waste of system resources and may prevent efficient use of CPU caches (thread stacks etc.).

Attached is a patch that provides a workaround until XNIO can provide a real solution for the problem.