[server] fix name and add docstring to connection initialization functions

[server] always monkeypatch QuerierHelper to handle dates and times on SQLite The patch prevents problems due to a SQLite bug causing some values to be returned as strings instead of properly typed objects. This should probably be improved to use SQLite connection hooks. But the major improvement is that the monkeypatch is now done all the time, and not just for tests.

[testlib] put postgres test database in test/data/database directory with other (sqlite) test databases. Closes #9240765 Simplify .hgignore in passing.

[dataimport] make eids_seq_range as massive store instance attribute again I changed it to a class attribute recently, but on further thought (and prompting from Adrien) it seems to make more sense as an instance attribute.

[dataimport] stop supporting None as eids_seq_range It couldn't work as-is because entities_id_seq is not a sequence object.

[dataimport] remove eids_seq_start attribute from massive store Instead provide a utility function to reset the eid sequence.

[dataimport] massive store in slave mode shouldn't flush metadata The metadata insertion is to be done only by the master-mode store.

[dataimport] check that MassiveObjectStore restores the db schema properly Ideally we'd use "pg_dump --schema-only" to compare before/after, but when restoring a constraint such as: cstr48f9ceae537d68859b62c601681c8d3e CHECK (((cw_type)::text = ANY ((ARRAY['normal'::character varying, 'auto'::character varying])::text[]))) postgres will convert the text cast on array to a cast on each cell of array cstr48f9ceae537d68859b62c601681c8d3e CHECK (cw_type::text = ANY (ARRAY['normal'::character varying::text, 'auto'::character varying::text]))

[dataimport] split out listing indices/constraints from getting the corresponding query

[dataimport] massive_store: do not drop constraints multiple times It is now done in __init__.

[dataimport] massive_store: do not drop constraints with `cascade` since it breaks things We want to be able to restore constraints at the end of the import. If they are silently dropped due to a CASCADE, we can't do that.

[dataimport] massive_store: drop all constraints in __init__ and restore all in finish This should avoid dependency issues when dropping specific constraints (e.g. when dropping a primary constraint referenced by a foreign key constraint). We only do this in master mode, slave mode stores are supposed to work with an initialized db already.