Separate repository mirroring into an Engine and provide bin/repository mirror

Summary:
Ref T4338. Currently, there's no diagnostic command to execute mirroring (so I can't give users an easy command to run), and it's roughly the last piece of real logic left in the PullLocal daemon.

Separate mirroring out, and provide bin/repository mirror.

Test Plan:

• Ran bin/repository mirror to mirror a repository.
• Ran PullLocalDaemon and verified it also continued mirroring normally.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4338

Differential Revision: https://secure.phabricator.com/D8066