Don't backup if draft `modified` timestamp hasn't advanced

[jwodder]

Closes #239.

This PR makes the following changes:

• When backing up the draft version of a Dandiset, the draft's modified timestamp is now stored in .dandi/assets-state.json at the end of the backup. Intermediate commits made along the way store the modified timestamp of the most recent asset at that point.
• When starting a backup of the draft version of a Dandiset, if .dandi/assets-state.json exists and the timestamp within is not earlier than the draft's current modified date, no backup is performed.
• When backing up a published version of a Dandiset, the version's created timestamp is now stored in .dandi/assets-state.json.
• A --verify-timestamps flag has been added to update-from-backup; when given, if a backup would be skipped due to timestamps, the backup is instead run, and an error occurs if anything would be changed.
• If there is any asset metadata that needs to be "garbage collected," an error is emitted unless the --gc-assets flag is passed to update-from-backup.

[codecov[bot]]

Codecov Report

Merging #245 (bbd69c8) into draft (a7b7215) will decrease coverage by 0.39%. The diff coverage is 69.86%.

@@            Coverage Diff             @@
##            draft     #245      +/-   ##
==========================================
- Coverage   78.30%   77.90%   -0.40%     
==========================================
  Files          14       14              
  Lines        1954     2014      +60     
  Branches      323      341      +18     
==========================================
+ Hits         1530     1569      +39     
- Misses        301      314      +13     
- Partials      123      131       +8     

Impacted Files	Coverage Δ
tools/backups2datalad/zarr.py	78.20% <20.00%> (-2.96%)	:arrow_down:
tools/backups2datalad/syncer.py	82.25% <42.85%> (-5.68%)	:arrow_down:
tools/backups2datalad/asyncer.py	80.11% <69.23%> (-0.60%)	:arrow_down:
tools/backups2datalad/datasetter.py	76.78% <85.71%> (+0.11%)	:arrow_up:
tools/backups2datalad/__main__.py	63.58% <100.00%> (+1.15%)	:arrow_up:
tools/backups2datalad/adataset.py	80.80% <100.00%> (+1.34%)	:arrow_up:
tools/backups2datalad/config.py	91.54% <100.00%> (+0.24%)	:arrow_up:
tools/backups2datalad/util.py	79.69% <100.00%> (+0.30%)	:arrow_up:

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[yarikoptic]

Coolio, thank you! I will add a cron job to do a run with --verify-timestamps once a day