Extract Executor::KeepAlive constants to a base class
Extract Executor::KeepAlive constants to a base class

Summary: [Folly] Extract Executor::KeepAlive bitmask constants to a non-template base class so that they can be used from elsewhere.

Reviewed By: LeeHowes

Differential Revision: D16617648

fbshipit-source-id: eb3ceddfa8c492bb4ddd7de0046bb5c2c6b4e762

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