Implement a MySQL-backed global lock

Summary: Implementation is a little crazy but this seems to work as advertised.

Test Plan: Acquired locks with "lock.php". Verified they held as long as the process reamined open and released properly on kill -9, ^C, etc.

Reviewers: nh, jungejason, vrana, btrahan, Girish, edward

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1400

Differential Revision: https://secure.phabricator.com/D2864