Factor out distributed SMC logic

[jeromekelleher]

Bring SMC k logic together in one place as part of #2294

[codecov]

Codecov Report

Attention: Patch coverage is 80.80808% with 19 lines in your changes missing coverage. Please review.

Project coverage is 91.15%. Comparing base (8d65577) to head (3faea4e).

Files with missing lines	Patch %	Lines
lib/msprime.c	80.80%	11 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2303      +/-   ##
==========================================
+ Coverage   90.85%   91.15%   +0.30%     
==========================================
  Files          20       20              
  Lines       11993    11889     -104     
  Branches     2330     2300      -30     
==========================================
- Hits        10896    10838      -58     
+ Misses        606      576      -30     
+ Partials      491      475      -16     

Flag	Coverage Δ
C	91.15% <80.80%> (+0.30%)	⬆️
python	98.68% <ø> (ø)

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