This patch adds the `waitforslot` argument to apply_async. If set to `True`, apply_async will not return until there is a worker available to process the job.

This is implemented by a semaphore that is released by the result handler whenever a new result is ready. The semaphore is also released
when the supervisor (worker_handler) finds a worker process that has been
unexpectedly terminated.

This is already in use by Celery 2.0, which ships with its own modified version of multiprocessing.pool.

I'm not sure about the name ``waitforslot``, I think I may even hate it, but haven't been able to come up with a better name for it yet.