Fixes for export and drop issues

[nellh]

Fixes a logic error in the task and reduces concurrency to reduce the chance of out of memory restarts.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 43.01%. Comparing base (c523a9f) to head (945a537).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
services/datalad/datalad_service/tasks/publish.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3659   +/-   ##
=======================================
  Coverage   43.01%   43.01%           
=======================================
  Files         636      636           
  Lines       33901    33901           
  Branches     1515     1515           
=======================================
  Hits        14583    14583           
  Misses      19179    19179           
  Partials      139      139           

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