Provide executor getter for ExecutorLoopController
Provide executor getter for ExecutorLoopController

Summary: This change enables to acquire the executor from the current FiberManager when using ExecutorLoopController (which is already possible to do with the EventBase's loop controller).

Reviewed By: andriigrynenko

Differential Revision: D14515799

fbshipit-source-id: 3b9e48450bf58ea94bebab50dbbcfd7720f90d61

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