1/ it's a 'private' class (no reason to use it from any other place
than this module), and
2/ it's an 'abstract' base class
This is needed (at least one of the 2 parts of the rename, but both make
sense semantically for the reasons listed above) to make it compatible
with the new auto register mechanism introduced in swh-scheduler 0.0.38;
note however that this requires D868 to behave properly.