Rewrite `ThreadPoolWrapper` to use atomic pointer

[torkleyy]

There seems to be no need to have a lock around an option wrapping an Arc - since we have an (allocated) Arc, we can just use its raw pointer for AtomicPtr, and swap it out whenever the pool should be changed.

cc @AndreaCatania

[AndreaCatania]

Yep, AtomicPtr is enough

[AndreaCatania]

If you want I can do this change, let me know