Summary:
[Folly] Use `relaxed_atomic` in `IOThreadPoolExecutor`.

For the `nextThread_` field, which may be mutated within `pickThread` in any number of threads concurrently. While `pickThread` is only ever called with a shared lock held, the shared lock does not actually help with concurrent mutations.

Since the mutations of `nextThread_` by `pickThread` is always a `std::atomic::fetch_add` with `std::memory_order_relaxed`, and since that case is a bit more nicely solved with `folly::relaxed_atomic`, switch to that.

Reviewed By: ot

Differential Revision: D6290451

fbshipit-source-id: e473fa6321e462c9b3df9d2bd9650967cca3444a