improve massive deletion performance
change hooks.integrity._DelayedDeleteOp implementation to give it a chance of
processing the entities by chunks of reasonnable size (500 entities at a time)
adapt ssplanner.DeleteEntitiesStep to call a variant of glob_delete_entity with several entities.
That variant calls all the before_delete_entities hooks in one go, then
performs the deletion, and then calls all the after_delete_entities hooks. The
deletion is performed by grouping together entities by etype and by source.
adapt the HooksManager to call the hooks on a list of entities instead of on a single entity.
adapt the sources to be able to delete several entities of the same etype at once.
changed the source fti_(un)index_entity methods to fti_(un)index_entities taking a collection of entities.