feat: try-improve concurrency tuner

[ritchie46]

On discord users mentioned a deadlock, so never await the rwlock.

We also don't tune if users set the budget manually. This will help testing if the tuner is the culprit.

[codecov]

Codecov Report

Attention: Patch coverage is 37.50000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 80.94%. Comparing base (5a5abdc) to head (c6ea4c4).

Files	Patch %	Lines
crates/polars-io/src/pl_async.rs	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14827      +/-   ##
==========================================
- Coverage   80.95%   80.94%   -0.01%     
==========================================
  Files        1327     1327              
  Lines      172080   172086       +6     
  Branches     2452     2452              
==========================================
- Hits       139305   139303       -2     
- Misses      32305    32312       +7     
- Partials      470      471       +1     

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