respect users OMP limits with setDTthreads

[ben-schwen]

Closes #7165

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.13%. Comparing base (bc7f5af) to head (5921b46).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7389   +/-   ##
=======================================
  Coverage   99.13%   99.13%           
=======================================
  Files          85       85           
  Lines       16610    16610           
=======================================
  Hits        16466    16466           
  Misses        144      144           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions]

• HEAD=omp_limits stopped early for fread disk overhead improved in #6925
  

Generated via commit 5921b46

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	5 minutes and 5 seconds
Installing different package versions	9 minutes and 56 seconds
Running and plotting the test cases	2 minutes and 36 seconds

[MichaelChirico]

Why do we need to have initDTthreads() and setDTthreads() separately? Feels like they should share even more logic

[ben-schwen]

Well setDTthreads calls initDTthreads as helper. But they have separate functions with startup initalization and runtime configuration.