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
Instead of writing directly to file_hashes.txt, hash_workers now are
using a local arena, writing everything once at the end
using #pragma once to ensure that a given header file is included only
once in a single compilation unit
