planner: simplify accounting during migration

[csegarragonz]

There was a bug in the accounting logic during a DIST_CHANGE request. At this point, we need to release the slots of the to-be migrated function, and claim them in the migrated-to hosts.

The current logic assumed that, given two decisions, they would always start being similar, and then diverge (or even diverge all the way through).

This is not the case. Take for example the following situation where A has 5 slots:

Old, New
B, A
B, A
B, A
C, A
C, A
A, C
A, C

Using our accounting logic, for a while we would have 7 slots assigned to A. This would break our assertions (did not catch it because it only happens in Release builds), but was breaking the MPI port assignation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.18%. Comparing base (da5fee2) to head (0145420).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #427      +/-   ##
==========================================
- Coverage   82.25%   82.18%   -0.08%     
==========================================
  Files         115      115              
  Lines        7586     7584       -2     
==========================================
- Hits         6240     6233       -7     
- Misses       1346     1351       +5     

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