Bust cache for draft files when file exists on S3

[nellh]

Fixes #3047. This cache was created with the key "HEAD" and the cached value would be returned if the file existed on S3. Instead this caches the draft with the commit hash. This does mean an extra worker API call for each file during a download, so added a short lived cache to avoid bulk file requests from repeating this request to the worker.

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 42.48%. Comparing base (cd5c1e0) to head (7cee72f).
Report is 2 commits behind head on master.

Files	Patch %	Lines
packages/openneuro-server/src/datalad/draft.ts	15.38%	11 Missing ⚠️
packages/openneuro-server/src/handlers/datalad.ts	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3052      +/-   ##
==========================================
- Coverage   42.48%   42.48%   -0.01%     
==========================================
  Files         601      601              
  Lines       37971    37980       +9     
  Branches     1113     1113              
==========================================
+ Hits        16131    16134       +3     
- Misses      21624    21630       +6     
  Partials      216      216              

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[effigies]

Makes sense to me.