[dataimport] add tests for SQLGenObjectStore (closes #9177565)
Stolen from the massive import store, with the irrelevant bits removed.
Remove redundant tests from test_pgstore.py.
[dataimport] separate entities table from other metadata in SQLGenObjectStore
'entities' needs to be updated first as every other table has references
to its rows. Remove use of thread-local storage in the process.
related to #9177565.