[etwist] Do not call repository's start_looping_tasks anymore and warn about this
We are about to drop this method from Repository class and replace it by a
blocking alternative. This is not compatible with how things currently work in
a Twisted server implementation. So do not start repository "looping tasks" in
Twisted server anymore and issue a warning about this.
If someone is interested in restoring the "all-in-one" behavior where the
repository runs within a Twisted server, they may start by implementing
repository looping tasks using a Twisted mechanism such as, e.g.,
http://twistedmatrix.com/documents/current/core/howto/time.html and eventually
provide the repository with a compatible scheduler instance so that is can
register its periodic tasks. At the moment, we lack resources to do this (and
maintain the Twisted server of CubicWeb in general).
Related to #17057223.