Summary: `BlockingWaitExecutor::add` (and similarly `WaitExecutor`) posts the baton while holding the queue lock. If these happen in two different threads, the `drive()` thread wakes up while the lock is still held; the `add()` thread may be descheduled before the lock is released, causing the other thread to have to wait (and possibly sleep) for a while.

Reviewed By: andriigrynenko

Differential Revision: D25814620

fbshipit-source-id: 07d3e9afdc4647e68f996e3f0a1c007fd1596611