We stop depending on the ListerBase implementation. The main hoop we're jumping
through is the config override mechanism in swh.lister.get_lister, as it's
really specifc to the ListerBase `override_config` argument, which is dropped in
pattern.Lister (in favor of explicit arguments at lister instantiation).
We implement a small shim in swh.lister.pattern.Lister to give
backwards-compatibility for the new pattern to get_lister.
This generic configuration override mechanism will probably be completely
removed when the configuration mechanism is reworked. We'll see.
This new pattern uses the lister support features introduced in swh.scheduler to
replace the database management done in previous iterations of the listers.
