Add a cache for the last seen event loop to avoid lookup cost
Add a cache for the last seen event loop to avoid lookup cost

Summary: As in title, skip lookup in `WeakKeyDictionary` for repeated `get_fiber_manager` calls on the same event loop

Reviewed By: pranavtbhat

Differential Revision: D32566214

fbshipit-source-id: 7a9305ef88538ae4ef422283bc1b8a4140b8ea33

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly