[IMP] de-commit-ify db/module install

Try to remove cr.commit (and rollback) from module and db install:

* put a savepoint around test data loading
* remove a bunch of commits sprinkled throughout
* remove rollback on data loading failure (assuming it bubbles up, the
  entire module's installation should be rolled back)
* add commit right before the tests are run, so they can run isolated
  and still see whatever was done when installing their module
* convert a few explicit closing to context managers

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