Praefect: SERVER scoped RPCs are redirected only to the primary of the first virtual storage

In current implementation all SERVER scoped RPC calls redirected to the primary gitaly node of the first virtual storage in the configuration list by the coordinator. And the only actually redirected call is FindRemoteRepository that will be migrated to the STORAGE scope level in !2234 (merged).
All the other RPCs are handled by the registered endpoints in praefect itself: ServerService with server service and RepositoryReplicas RPC with info service.
There are no more SERVER scoped RPC, so no any request with SERVER scope should reach praefect's coordinator code and be redirected somewhere.

/cc @zj-gitlab @jramsay