Improved ReservedThreads idle timeout

[sbordet]

After #1805, ReservedThreads are lazily allocated on demand.
If the server goes idle, these threads currently remain allocated.
It would be good if they can sit idle for a while and then be deallocated.

ReservedThreadExecutor currently use a circular buffer to hold ReservedThreads and there is a strong assumption in the code that ReservedThreads are used from a queue.

Implementing idle timeout means that when a ReservedThread may idle timeout it should be removed from an arbitrary index of the circular buffer, which would then have a hole that would then require other elements to be shifted, adjusting head and tail pointers. Perhaps just use a JDK queue ?

[gregw]

I've created a proposed fix in branch https://github.com/eclipse/jetty.project/compare/jetty-9.4.x-1806?expand=1

However, it will only shrink if the whole pool is idle for the period.

It may be better to have a stack of threads rather than a queue, so last in first out will be applied (good for data and instruction caches) and we can idle out unused threads in a moderately active reserved pool.

[gregw]

@sbordet I have updated the branch to use a stack of reserved threads. This thus is we have a low level of activity, a few reserved threads may be used (with hotter caches) and the threads on the bottom of the stack will timeout.

Review is needed!

[gregw]

@sbordet In branch (jetty-9.4.x-1806-ConcurrentReservedThreadExecutor](https://github.com/eclipse/jetty.project/compare/jetty-9.4.x-1806-ConcurrentReservedThreadExecutor?expand=1) I have reimplemented ReservedThreadExecutor to use a non blocking stack and to handle timeouts.