periodic timers should not cause leaking of folly::RequestContext
periodic timers should not cause leaking of folly::RequestContext

Summary:
we do have some periodic maintentenance tasks... if those are initialized
lazily on a requests handling path, they  will capture current rctx and keep it
it alive forever.

Reviewed By: yfeldblum

Differential Revision: D22859344

fbshipit-source-id: 3655d874dee22aaf74b4d80631d4a58e83539752

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