Add a global lock for the RepositoryPullDaemon

Summary: Allow multiple daemons to run without contention.

Test Plan: Ran multiple daemons simultaneously in "debug" mode, observed them acquiring (and sometimes failing to acquire) locks.

Reviewers: btrahan, jungejason, nh

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1400

Differential Revision: https://secure.phabricator.com/D2877