Fast path for UnboundedQueue::try_dequeue()?

https://github.com/facebook/folly/blob/7613938ff11c89c50f28a2c2bc98e6df3ea15ca5/folly/concurrency/UnboundedQueue.h#L775

We're doing a tokio-like runtime that uses an UnboundedQueue as the run queue, and a common thing to do is call try_dequeue(), and go into IDLE state when it returns None. The current try_dequeue implementation will make two unnecessary calls to `steady_clock::now()`, one taking ~20ns in our environment.

So should we add a fast path to try_dequeue(), which will reduce try_dequeue latency from ~46ns to ~3ns. I'd love to contribute an PR if we should.