praefect: Implement ability to force-route repo-scoped accessors to the primary
With reads distribution being enabled, requests would typically be routed to a random up-to-date node. There is situations though where not even up-to-date nodes would be able to serve a request, e.g. when object quarantine is in use.
This MR implements force-routing to the primary for the node-manager and per-repository routers. It's implemented a new GRPC metadata header "gitaly-route-repository-accessor-to-primary": if set, a repository-scoped accessors will always get routed to the primary.
Part of #3438 (closed)