Template KeepAlive with Executor type
Template KeepAlive with Executor type

Summary:
This allows using KeepAlive token as a pointer to an specific Executor sub-type (e.g. SequencedExecutor).
getKeepAliveToken() has to be a free function to achieve this goal.

Reviewed By: yfeldblum

Differential Revision: D7847884

fbshipit-source-id: 95dc62433d30212ceb9b9b2fc7d6b38cb5425311

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter