Fix wrongly check if the migrating slot in the source node

[git-hulk]

Kvrocks will return migrating_slot(s) instead of migrating_slot
after the slot range migration is supported. From the controller, we
need to allow both of them to be compatible with the old behavior.

This close #267.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.01%. Comparing base (6c56470) to head (e5d00fd).
⚠️ Report is 104 commits behind head on unstable.

Files with missing lines	Patch %	Lines
controller/cluster.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #269      +/-   ##
============================================
+ Coverage     43.38%   47.01%   +3.62%     
============================================
  Files            37       45       +8     
  Lines          2971     4222    +1251     
============================================
+ Hits           1289     1985     +696     
- Misses         1544     2029     +485     
- Partials        138      208      +70     

Flag	Coverage Δ
unittests	47.01% <25.00%> (+3.62%)	⬆️

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