UnboundedQueue: Change try_dequeue variants to fail only if the queue is empty

UnboundedQueue: Change try_dequeue variants to fail only if the queue is empty

Summary:
The current semantics of try_dequeue variants can cause problems in cases where based on external dependencies the consumer is guaranteed that the queue is not empty and depends on the success of try_dequeue. See https://github.com/facebook/wangle/commit/abd07a06cae09efcd4347a9a019d59ae8a013c3f

This diff changes the semantics so that try_dequeue operations fail only if the queue is empty.

Reviewed By: yfeldblum

Differential Revision: D6586156

fbshipit-source-id: 25d6085e28d3e24034ecf6a8bafab51c95464b01