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
Fix bug slots used before initialization,compare and swap is protecting
updating committed, but it is not protecting the memory initialization.
Adding atomic_flag commit_lock to protect against that
Fix bug multiple threads committing at the same time, fixed by using
atomic_flag commit_lock and re-checking committed after acquiring the
lock
Reorder helper functions
