Add a TODO for "solBnkNewFlds" threadpool

[HaoranYi]

Problem

Add a TODO commentsfor "solBnkNewFlds" threadpool optimization.

We can do this after lt-hash feature is active.

Summary of Changes

Fixes #

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.8%. Comparing base (ccdcdbe) to head (ab62f7e).

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #6584     +/-   ##
=========================================
- Coverage    82.8%    82.8%   -0.1%     
=========================================
  Files         849      849             
  Lines      379032   379039      +7     
=========================================
- Hits       313911   313897     -14     
- Misses      65121    65142     +21     

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[brooksprumo]

I'm usually against adding TODOs into the repo. But we're close to activating the lt hash features on all clusters, and then I can clean up the featurization code, which'll then allow addressing the TODO.