fix: move IO tasks off of CPU runtime in merge_insert

[wjones127]

Fixes #3419

We were using the CPU_RUNTIME, but there's a lot of IO happening here, and it wasn't configured to do that. Instead, I moved the operations onto the main runtime so it can perform the IO.

✅ I've testing this manually against S3 with the repro.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.82%. Comparing base (58c5e27) to head (26d885f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3420   +/-   ##
=======================================
  Coverage   78.81%   78.82%           
=======================================
  Files         250      250           
  Lines       91306    91305    -1     
  Branches    91306    91305    -1     
=======================================
+ Hits        71963    71970    +7     
+ Misses      16390    16380   -10     
- Partials     2953     2955    +2     

Flag	Coverage Δ
unittests	78.82% <100.00%> (+<0.01%)	⬆️

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