Fix a race condition in the lockfree hash table

We didn't know, but Preshing's hash table has almost full-featured
CAS functionality. It is available via a mutator, which reads the
value and doesn't allow writing back if the source value is has changed.

The actual race condition of the previous implementation happened
on 3+ cores system. While two threads were fighting for putting their
tile to the table, some other (third) thread could take one of their
tiles, initialize it and use for writing. Obviously, this tile could
be pushed out of the table as a result of the fight, so the written
data would just get lost, causing white holes in the image.