Now we have 3 different API compatible MPMC queues that we can swap with
swapping the header.
mt_mpmc.h, a blocking queue that uses a mutex/critical section.
lf_mpmc.h, a lock free queue that uses atomics.
sm_mpmc.h, a hybrid queue that uses atomics and a semaphore to block
when the queue is empty.
In this program, for max performance it is recommanded to use sm_mpmc.h
or mt_mpmc.h, they are designed to avoid busy waiting which frees more
CPU time to do useful work.
Making the MPMC queue support when producers are consumers at the same
time by adding a variable work, mpmc_push_work() that increments work
and mpmc_task_done() that decrements work, and if work = 0 calls
mpmc_producers_finished() that pushes poinsons to wake up sleeping
threads and make them return NULL
Replacing DirQueue, a queue growable with realloc with the MPMC queue
