Add 30 second timeout to dataset resolver

[nellh]

This avoids aggregate queries (search) from hanging forever if one result cannot be loaded in the 30 second deadline.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 65.28%. Comparing base (5495968) to head (f71d40f).

Files	Patch %	Lines
...kages/openneuro-server/src/utils/promiseTimeout.ts	93.54%	2 Missing ⚠️
.../openneuro-server/src/graphql/resolvers/dataset.ts	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3012      +/-   ##
==========================================
+ Coverage   65.24%   65.28%   +0.04%     
==========================================
  Files         382      384       +2     
  Lines       24610    24648      +38     
  Branches      885      891       +6     
==========================================
+ Hits        16056    16092      +36     
- Misses       8554     8556       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.