[#1725] Remove useless code

This was moved to the Datastore plugin, so there's no need to be here anymore.
[#1725] Add test to guarantee that the filters received are in a dict
[#1725] Add tests asserting that the IDataStore.where() works for delete

It should be used both on datastore_create and datastore_delete.
[#1725] Add delete_data() extension point and remove where()

@amercader and I agreed that, even though in most cases both select_data() and
delete_data() would create the WHERE clauses with the same code, it's easier to
understand the WHERE being created inside each one of those, not on a separate
where() extension point.
[#1725] Rename IDataStore -> IDatastore

This makes it consistent with the plugin name (DatastorePlugin).
[#1725] Reorder arguments on search_data() and delete_data()

This keeps our current 3 extensions points, search_data(), delete_data(), and
validate_query(), with the same initial 3 arguments, in the same order. The
interface becomes a bit more consistent.