record workers in a set
...instead of a queue.

That way when an idle worker is restarted (and sends an 'idle' message
to the pool), it won't end up in the pool twice.

Note that we always hand out work to the first worker in the
ordset. That is actually more efficient than the round-robin strategy
we had with the queue since it keeps a smaller number of workers busy
while others can hibernate.
track workers by Pid instead of name
monitor workers
In a way this is a damage limitation exercise...

When a worker dies while working it is not in the pool anyway, so
noticing its death is a no-op.

When a worker dies while idle, it will not return to the pool when we
next submit work to it. But obviously the submitted work won't be
carried out, and the submitter will get an error (unless they
submitted the work asynchronously).

All the monitoring does is reduce the likelihood of the latter
happening. It cannot eliminate it though since the worker may die just
as work was being submitted to it.
Merge bug 26123
stable to default