If the worker pool is configured too small, it can deadlock.

Recovering from this would require a lot of complicated code, to
discover queued but unscheduled tasks which can be cancelled
(because the client went away or otherwise) and code to do the
cancelling etc. etc.

But fundamentally either people configured their pools wrong, in
which case we want them to notice, or they are under DoS, in which
case recovering gracefully is unlikely be a major improvement over
a restart.

Instead we implement a per-pool watchdog and kill the child process
if nothing has been dequeued for too long.

Default value 10 seconds, open to discussion.

Band-aid for:	#2418

Test-case by:	@Dridi