New closure flag to mark the end of a private_queue.
Reenabled freeing of executors.
Reorganized private_queue and processor modules.

The relationship between them now puts more into the private queue,
which seems to be the more logical location.
Updating wait_query to use new style.

It's still a little outdated though, perf_mutex is the latest
template with the private queue shutdown.
Adding perf_mutex performance test.

It currently reveals a deadlock bug somewhere.