[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.

[auth] pass `repo` instead of `vreg` to SessionManager and AuthenticationManager Those object actually need a repo object to connect and create new session object. They used to retrieve the repo object using ``vreg.config.repository()``. I'm trying to sanitise the initialisation stack and get ride of ``config.repository()``, a function that mix factory and cache. Retrieving the vreg from the repo is trivial (``repo.vreg``) so we now pass the repo object instead of the vreg.

web/application: instantiate the repository outside of CubicWebPublisher This improves decoupling and allows ``CubicWebPublisher`` user to choose which argument are used during ``Repository`` instantiation. In particular is allows caller to provide a ``TaskManager``. The `vreg` argument of publisher is made mandatory. See CubicWebPublisher docstring for details. Related to #2249513

remove vreg argument to CWPublisher The passing a vreg to CWPublisher was used by test to allow reused of an existing vreg. We dropped this feature two commit ago and can safely removes it. Removing this argument allows later cleanup and improvement on the CWPublisher and Repository front. closes #2944813