R1:1c73ad6a1bb0

Make repository daemon locks more granular and forgiving

Summary:
Ref T4292. Currently, we hold one big lock around the whole `bin/repository update` workflow.

When running multiple daemons on different hosts, this lock can end up being contentious. In particular, we'll hold it during `git fetch` on every host globally, even though it's only useful to hold it locally per-device (that is, it's fine/good/expected if `repo001` and `repo002` happen to be fetching from a repository they are observing at the…