R1:4244cad99073

Move toward multi-master replicated repositories

Summary:
Ref T4292. This mostly implements the locking/versioning logic for multi-master repositories. It is only active on Git SSH pathways, and doesn't actually do anything useful yet: it just does bookkeeping so far.

When we read (e.g., `git fetch`) the logic goes like this:

- Get the read lock (unique to device + repository).
- Read all the versions of the repository on every other device.
- If any node has a newer version:
- Fetch the newer…