Provide an ad-hoc maintenance lock for clustered repositories

Summary: Ref T13614. Provide "bin/repository lock" to temporarily lock repositories for manual maintenance.

Test Plan:

• Read instructions.
• Used bin/repository lock according to the instructions.
• Saw Storage tab in Diffusion report lock held during maintenance, released after it completes.
• Saw "maintenance" push log generated and repository version bump.
• Tried to lock some invalid repositories.

Maniphest Tasks: T13614

Differential Revision: https://secure.phabricator.com/D21671