[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.

[repo] move repo stats to Service (closes #2951067) We currently add a new method on repo that anyone can call. Call service allows to reach the same result with: * Restricted access to manager only (thanks to selection on services), * Less unrelated code on the repo. * No need to fetch a reference to a repo object from 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.stats()) is deprecated.

[service] drop the asynchronous execution possibility Call_service was able of both sync and sync execution making the API confusing. There is not user of the async case. We drop the async argument in favor of synchronous execution only. This makes call_service the official API to call server side code from the client side. This is a remplacement for the usual monkey patching of the repo object. The zmq notification bus is a solid alternative for codes that needs to start an async execution.