- 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
- Currently we just use 1.5x and 2x the normal preview delay; we could
change this in the future.
- Don't try to capture the current state; just use a flag to denote
exit status.
- Show (end) when exiting
