Add a filesystem locking primitive

Summary:
This is a simple wrapper around flock() that gives us sensible semantics.

These locks behave well and are released when the process exits, even if it kill -9'd, so we don't need to do any kind of lock recovery or timeouts. This is really nice since handling that stuff is awful.

Test Plan: Ran unit tests, which provide reasonably extensive coverage of behavior.

Reviewers: jungejason, nh, vrana

Reviewed By: nh

CC: aran

Maniphest Tasks: T1400

Differential Revision: https://secure.phabricator.com/D2853