[massive store] Ensure temporary metadata table get dropped even if there is some integrity errors while inserting entity values or setting back index/constraints, or any other errors. In case of exception, rollback to ensure we're not committing undesired intermediate state.

[test] Stop using lgc.testlib.unittest_main in dataimport's test_sqlgenstore.py

[test/cleanup] Cleanup flake8 errors in dataimport's test_sqlgenstore