[BUG] clean up ray scheduler threads after computing partial results

[samster25]

• adds stop_plan method on the Ray Scheduler when is called when the RayRunner.run_iter goes out of scope.
• This fixes the issues where we have a hanging thread at the end of execution
• closes: Ray deadlocks on shutdown after Daft execution #1591

[codecov]

Codecov Report

Merging #1597 (d70c3dd) into main (ef4d2fd) will decrease coverage by 0.27%.
Report is 2 commits behind head on main.
The diff coverage is 75.75%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1597      +/-   ##
==========================================
- Coverage   85.21%   84.94%   -0.27%     
==========================================
  Files          54       54              
  Lines        5180     5207      +27     
==========================================
+ Hits         4414     4423       +9     
- Misses        766      784      +18     

Files	Coverage Δ
daft/runners/ray_runner.py	89.79% <75.75%> (-0.85%)	⬇️

... and 9 files with indirect coverage changes

[jaychia]

Do we need to call stop_plan here after the while loops finish to clean up?

[samster25]

stop plan is called by the main thread to make this loop (in the background thread) complete and could be a race condition since the results may not be collected yet.