Bug fixes in the lock free mpmc queue

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

platform.h

@@ -117,6 +117,7 @@ typedef struct {
 
   atomic_size_t committed;
   size_t commit_step;
+  atomic_flag commit_lock;
 
   MPMCSlot *slots;
 } MPMCQueue;