Call ServiceHandler::setServer with nullptr after server stops
Call ServiceHandler::setServer with nullptr after server stops

Summary: Accessing the server past this point is likely an error - it may even be destroyed since the handler can outlive the server.

Reviewed By: yfeldblum

Differential Revision: D31490847

fbshipit-source-id: b4f61fc0c02f68ba85e6f0c00f4327b88f298980

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