[repo] move repo.gc_stats to Service API (closes #2951068)
We currently have a method on the repo that anyone can call. `call_service`
gives a similar thing, modulo:
* can be restricted access to manager only (thanks to selection on services),
* less unrelated code on the repository class,
* no need to fetch a reference to a repo object from the client side
* (`req._cnx.repo` is not an API and config.repository() is on its way out).
The old way to access this information (repo.gc_stats()) is deprecated.
