Cache file digests and check for change in digest when uploading

[jwodder]

Closes #389.

This PR reverts #388.

[codecov]

Codecov Report

Merging #391 (49fbcf0) into master (48679c6) will decrease coverage by 0.00%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #391      +/-   ##
==========================================
- Coverage   82.39%   82.38%   -0.01%     
==========================================
  Files          55       55              
  Lines        5708     5711       +3     
==========================================
+ Hits         4703     4705       +2     
- Misses       1005     1006       +1     

Flag	Coverage Δ
unittests	82.38% <93.75%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dandi/dandiapi.py	87.21% <87.50%> (-0.52%)	⬇️
dandi/support/digests.py	100.00% <100.00%> (ø)
dandi/upload.py	76.00% <100.00%> (-0.06%)	⬇️

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[yarikoptic]

Looks good, thank you @jwodder!