Add safety around popping from GreenletSemaphore timeout set
Add safety around popping from GreenletSemaphore timeout set

Summary:
Saw an exception around removing from this set in prod this morning. Not
entirely sure how that'd happen, but easy to defend against.

Test Plan: Ran unit tests

Reviewers: david, andrew, sean

Subscribers:

Differential Revision: https://phab.wish.com/D35140

ContextLogic/mongoengine,ContextLogic/mongoengine