[dataimport] turn iid_maxsize into a class attribute

[dataimport] turn eids_seq_{start,range} into class attributes Only the master-mode store will actually use eids_seq_start to reset the eid sequence.

[dataimport] remove replace_sep parameter from massive store It is unused since changeset efbbf1e93a04 "[dataimport] Properly escape strings sent to COPY FROM"

[dataimport] remove pg_schema parameter from massive store It can be recovered from the system source's configuration.

[dataimport] remove drop_index parameter from massive store "drop_index=False" also implied not dropping any constraints, in particular foreign keys, which meant any attempt to import entities would fail, because we only add metadata (the entities table) after the entity's insertion.

[dataimport/test] update call to ucsvreader Silences warning: [3.20] 'separator' kwarg is deprecated, use 'delimiter' instead

[dataimport] remove deprecated methods from massive store This code is new in cubicweb, no reason for it to carry deprecated stuff from the start.

[dataimport/test] use store.finish instead of deprecated store.cleanup

[dataimport] drop no more used parameter on massive store

[dataimport] methods that modify in-place shouldn't return value Also mark those methods private and drop some useless comments in passing. Related to #5414760

[dataimport] implement new store API on massive store and deprecate the old one. Related to #5414760

[dataimport] .finish method was missing from (NoHook)RQLObjectStore

[massive store] identity_relation should not be considered, is_relation seems missing The "identity_relation" table should not even exist.

[dataimport/test] use the right assert methods instead of assertTrue with a comparison

[dataimport] import range from six.moves