R1:e0a8cac703d1

When no master database is configured, automatically degrade to read-only mode

Summary: Ref T4571. If `cluster.databases` is configured but only has replicas, implicitly drop to read-only mode and send writes to a replica.

Test Plan:
- Disabled the `master`, saw Phabricator automatically degrade into read-only mode against replicas.
- (Also tested: explicit read-only mode, non-cluster mode, properly configured cluster mode).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4571

Differential…