ENH: upload: --existing=overwrite-metadata to upload fresh metadata even if file already there

[yarikoptic]

To facilitate cases where we want to ask users to reupload while using newer dandi-cli which would extract metadata differently

Closes #105

TODOs:

• Development safeguard (DANDI_UPLOAD_SAME_MUST_EXIST env var) for paranoids so if we operate under assumption that we have up to date local data files, we do not mistakenly reupload (outdated) data.
• test on sample cases to verify correct operation (see e.g. incomplete info for a file #349 as one of the target use cases)

[codecov]

Codecov Report

Merging #355 (91b2af7) into master (53da9e9) will decrease coverage by 0.34%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master     #355      +/-   ##
==========================================
- Coverage   82.15%   81.81%   -0.35%     
==========================================
  Files          55       55              
  Lines        5454     5482      +28     
==========================================
+ Hits         4481     4485       +4     
- Misses        973      997      +24     

Flag	Coverage Δ
unittests	81.81% <42.85%> (-0.35%)	⬇️

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

Impacted Files	Coverage Δ
dandi/cli/cmd_upload.py	88.88% <ø> (ø)
dandi/upload.py	66.66% <42.85%> (-3.19%)	⬇️
dandi/dandiapi.py	85.22% <0.00%> (-0.99%)	⬇️

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[yarikoptic]

"tested" in real life, managed to delete file with some version before this one... so I better wait to have done more testing or if I add some unittest to guarantee that there is no side effects... and we might not need it in the long run with a new API server since if data blob is the same - we should not reupload it.

[yarikoptic]

I will close it for now ... can resurrect if the need arises again and would need to do it also for dandi-api interface