mcs: fix datarace in keyspace group

[lhy1024]

What problem does this PR solve?

Issue Number: Close #6721

What is changed and how does it work?

checkTSOMerge and checkTSOSplit will read from kgm.getKeyspaceGroupMeta

finishMergeKeyspaceGroup and finishSplitKeyspaceGroup will update kgm

so just return a copy to avoid data race

Check List

Tests

• Unit test

Release note

None.

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.06 ⚠️

Comparison is base (fa721e7) 74.57% compared to head (6c7c945) 74.52%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6722      +/-   ##
==========================================
- Coverage   74.57%   74.52%   -0.06%     
==========================================
  Files         416      416              
  Lines       43595    43599       +4     
==========================================
- Hits        32511    32492      -19     
- Misses       8193     8213      +20     
- Partials     2891     2894       +3     

Flag	Coverage Δ
unittests	74.52% <100.00%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/tso/keyspace_group_manager.go	79.52% <100.00%> (+0.12%)	⬆️

... and 37 files with indirect coverage changes

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[lhy1024]

It will be fix in #6723