migration: fix log for slot migration success

[travisshivers]

Previously, the code was trying to log the migrated slot after calling ClearMigrateState, which set the migrated slot to -1. Instead, capture the slot number so we can properly log it later.

[torwig]

@travisshivers Thank you for your vigilance.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.25%. Comparing base (6c56470) to head (4a371f3).
⚠️ Report is 100 commits behind head on unstable.

Files with missing lines	Patch %	Lines
controller/cluster.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #277      +/-   ##
============================================
+ Coverage     43.38%   46.25%   +2.87%     
============================================
  Files            37       45       +8     
  Lines          2971     4287    +1316     
============================================
+ Hits           1289     1983     +694     
- Misses         1544     2096     +552     
- Partials        138      208      +70     

Flag	Coverage Δ
unittests	46.25% <0.00%> (+2.87%)	⬆️

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