Add missing singleton shutdown handling
Add missing singleton shutdown handling

Summary:
This will become more important once we have a hard dep
on folly, and is a bit of a gap today: we set up the singleton
infra, but don't tear it down when we terminate the program.

This adds the shutdown via SCOPE_EXIT.

Refs: https://github.com/facebook/watchman/pull/636

Reviewed By: simpkins

Differential Revision: D9396879

fbshipit-source-id: 186381b7cdc76b5d66efb3b12a9668e5a64c38e6

facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,wez/watchman,wez/watchman,nodakai/watchman,wez/watchman,facebook/watchman,wez/watchman,wez/watchman,wez/watchman,wez/watchman,nodakai/watchman,wez/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,wez/watchman