Gitaly cleanup

Some cleanup in Gitaly code.

• assume that gitlab-rails sends GitalyAddress. Valid because of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10447
• assume that gitlab-rails sends a fully populated Repository message. Valid because of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10343
• use pre-built Repository message from API responses. This is something we missed in https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/143

assigned to @nick.thomas

resolved all discussions

assigned to @jacobvosmaer-gitlab

@jacobvosmaer-gitlab mostly LGTM, just one (possibly paranoid) performance query

assigned to @nick.thomas

resolved all discussions

approved this merge request

merged

mentioned in commit db9743d1