Permits to batch update content values (with or without optional new columns).
Limited to contents in the table content. The contents in
skipped_content are not dealt with as we cannot recompute hashes on
those since we don't have them in the objstorage.
Related T692
Note:
- There are limits to the current implementation as per the todo
description in the code (swh.storage.content_update).
- I have a problem when running the tests for now. Individually, as a
set of 2 tests, the new tests run fine. When running all the tests,
the one with the new columns hang and i don't know why yet (working on
it). This does not block the review though.