[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.
What's new in CubicWeb 4.0?
============================
API changes
-----------
* ``RepositorySessionManager.postlogin`` is now called with two arguments,
request and session. And this now happens before the session is linked to the
request.
* ``SessionManager`` and ``AuthenticationManager`` now take a repo object at
initialization time instead of a vreg.
* The ``async`` argument of ``_cw.call_service`` have been dropped. All call are
now synchronous. The zmq notification bus looks like a good replacement for
most async usecase.
* ``repo.stats()`` is now deprecated. The same information are available through
a service (``_cw.call_service('repo_stats')``)
* ``repo.gc_stats()`` is now deprecated. The same information are available through
a service (``_cw.call_service('repo_gc_stats')``)
Deprecated Code Drops
----------------------
* The ldapuser source has been dropped. ldapfeed is the only official source
remaining for ldap.
* session.hijack_user mechanism has been dropped.