We were (ab)using the bulk update routine to do deletions, but that
code was really intended to be used for finding&replacing, where an
exact match is not a requirement.
- fetch sfld and csum when fetching notes, to make it cheaper
to write them back out unmodified
- make `fields` private, and access it via accessors, so we can
still catch when fields have been mutated without calling
prepare_for_update()
- fix python importing code passing a string in as the checksum
- backend routines should contain minimal logic, and should call
into a routine on the collection
- instead of copying the giant-string approach the Python code was taking,
we use a HashSet to keep track of seen tags as we loop through the
notes, which should be more efficient
See https://github.com/ankitects/anki/pull/900#issuecomment-758284016
- Leave tag names alone and add the collapsed and config columns to the tags table.
- Update The DB check code to preserve the collapse state of used tags.
- Add a simple test for clearing tags and their children
