R1:51cb7a3db9e8

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…