Watchman Service:

-1- start an instance of watchman per-working directory
-2- register every source files to watchman:
    watchman will trigger the Falcon's client command to mark files dirty

Summary: Watchman service

Test Plan: start daemon, compile, edit, compile

Reviewers: adrienconrath

Reviewed By: adrienconrath

Subscribers: NicolasDP

Maniphest Tasks: T6

Differential Revision: http://91.121.199.76/D27