Reduce KeepAlive object size to 1 pointer
Reduce KeepAlive object size to 1 pointer

Summary: Use a single pointer to store Executor pointer and dummy flag.

Reviewed By: yfeldblum

Differential Revision: D7857087

fbshipit-source-id: 529fbfdc6ad14954f5bdd9bef3eb1e335b233ec9

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