Create longer thread destruction test.

This one involves shrinking the threadpool in such
a way that both idle and active threads are affected.

This test made me re-realize why the zombie state exists,
so I re-added it. We don't want to clog up the control
taskprocessor by waiting on active threads to complete
what they are doing. Instead, we mark them as zombies so
that when they are done, they can clean themselves up
properly.

Without the zombie state available, the new test actually
will deadlock.



git-svn-id: https://origsvn.digium.com/svn/asterisk/team/mmichelson/threadpool@377474 65c4cc65-6c06-0410-ace0-fbb531ad65f3