Run replication jobs against primary in read-only mode

Currently replication jobs do not run against a primary Gitaly node. We should allow this for a virtual storages' in read-only mode. This would make data recovery efforts easier following a failover as an administrator could then run praefect reconcile from the previous primary towards the new primary to recover the lost data.

After the virtual storage is again write enabled, we should not allow replication jobs against it anymore, as they would lose writes on the new primary.