fd_queue updates for the worker MPM

[Aaron Bannert <aa...@clove.org>]

I've created a new struct fd_queue_info_t that I believe can be used to
monitor the number of idle workers and make sure that we don't accept
more connections than we have available workers. I think by implementing
this we can also clear out some of the extra cruft in the queue, only
relying on a mutex to make sure we don't klobber the queue.

Most of this stuff could be optimized to use Ian's new atomic operators
stuff too. I'll post a patch in the next couple hours.

-aaron