Summary: Let `ThreadLocalPtr` member `reset()` synchronize with member type `Accessor` lifetime, preventing data races that could happen if `reset()` is called while another thread is within the `Accessor` critical section.

Reviewed By: yfeldblum

Differential Revision: D14878801

fbshipit-source-id: 4090c5749d08bee3fd9b50b23df6e716bc78c9c7