Close connection to watchman when canceling a watcher
Close connection to watchman when canceling a watcher

Summary:
When a Flow server exits, we cancel the file watcher. If we start up a new server we'll start a new file watcher.

Previously, we were leaking fds by not closing the old file watcher. This is a little better :)

Reviewed By: jbrown215

Differential Revision: D13060170

fbshipit-source-id: 7c263fe1cb4f3b6b473fd5a42ac530c9381a368b

facebook/flow,samwgoldman/flow,gabelevi/flow,mroch/flow,gabelevi/flow,gabelevi/flow,AgentME/flow,facebook/flow,samwgoldman/flow,facebook/flow,AgentME/flow,gabelevi/flow,facebook/flow,gabelevi/flow,samwgoldman/flow,popham/flow,AgentME/flow,mroch/flow,popham/flow,mroch/flow,popham/flow,gabelevi/flow,samwgoldman/flow,AgentME/flow,popham/flow,samwgoldman/flow,mroch/flow,popham/flow,facebook/flow,gabelevi/flow,facebook/flow,popham/flow,facebook/flow,mroch/flow,popham/flow,AgentME/flow,mroch/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow