Factor out distributed SMC logic

[jeromekelleher]

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.59375% with 21 lines in your changes missing coverage. Please review.

Project coverage is 91.13%. Comparing base (8d65577) to head (5a06136).

Files with missing lines	Patch %	Lines
lib/msprime.c	83.59%	12 Missing and 9 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2303      +/-   ##
==========================================
+ Coverage   90.85%   91.13%   +0.28%     
==========================================
  Files          20       20              
  Lines       11993    11868     -125     
  Branches     2330     2296      -34     
==========================================
- Hits        10896    10816      -80     
+ Misses        606      577      -29     
+ Partials      491      475      -16     

Flag	Coverage Δ
C	91.13% <83.59%> (+0.28%)	:arrow_up:
python	98.68% <ø> (ø)

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[jeromekelleher]

I'm going to merge this as it should be significant improvement in terms of code readability, and I think it also fixes a few memory leaks and performance issues. I'll follow up with issues for what remains in order to release the SMC(k) code.