perf: Reduce sharing in stringview arrays in new-streaming equijoin

[orlp]

This is a regression on some queries in PDS-H but gets rid of really problematic contention in others, so makes behavior more predictable & scalable.

The PR also includes some unused code for avoiding sharing in a chunked gather. From experimenting that was slower but I left it in because it might come in handy in the future for more experimentation.

[codecov]

Codecov Report

Attention: Patch coverage is 52.41935% with 59 lines in your changes missing coverage. Please review.

Project coverage is 79.07%. Comparing base (13186ba) to head (3981c0e).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...tes/polars-ops/src/chunked_array/gather/chunked.rs	69.04%	26 Missing ⚠️
crates/polars-core/src/frame/mod.rs	0.00%	14 Missing ⚠️
crates/polars-stream/src/nodes/joins/equi_join.rs	0.00%	11 Missing ⚠️
crates/polars-arrow/src/array/binview/mod.rs	0.00%	7 Missing ⚠️
crates/polars-expr/src/expressions/window.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21129      +/-   ##
==========================================
- Coverage   79.09%   79.07%   -0.03%     
==========================================
  Files        1592     1592              
  Lines      227174   227243      +69     
  Branches     2594     2594              
==========================================
+ Hits       179692   179693       +1     
- Misses      46886    46954      +68     
  Partials      596      596              

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