ServiceHealthPoller - survive vault destruction
ServiceHealthPoller - survive vault destruction

Summary: Make ServiceHealthPoller task survive vault destruction.

Reviewed By: praihan

Differential Revision: D37808329

fbshipit-source-id: ac06634bb7e25aaaac9f8a7a89312dd6a3786327

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift