enhance: Reduce mutex contention in datacoord meta

[bigsheeper]

1. Using secondary index to avoid retrieving all segments at GetSegmentsChanPart.
2. Perform batch SetAllocations to reduce the number of times the meta lock is acquired.

issue: #37630

[sre-ci-robot]

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[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 6 lines in your changes missing coverage. Please review.

Project coverage is 82.79%. Comparing base (2afe2ea) to head (1f8c0b5).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/datacoord/meta.go	33.33%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #38219      +/-   ##
==========================================
+ Coverage   80.91%   82.79%   +1.88%     
==========================================
  Files        1375     1083     -292     
  Lines      193645   167515   -26130     
==========================================
- Hits       156692   138699   -17993     
+ Misses      31418    23286    -8132     
+ Partials     5535     5530       -5     

Components	Coverage Δ
Client	74.58% <ø> (ø)
Core	∅ <ø> (∅)
Go	83.03% <72.72%> (+0.01%)	⬆️

Files with missing lines	Coverage Δ
internal/datacoord/compaction_policy_clustering.go	75.48% <100.00%> (-0.12%)	⬇️
internal/datacoord/compaction_policy_single.go	61.00% <100.00%> (-0.39%)	⬇️
internal/datacoord/compaction_trigger.go	83.97% <100.00%> (-1.17%)	⬇️
internal/datacoord/meta.go	89.27% <33.33%> (-0.37%)	⬇️

... and 319 files with indirect coverage changes

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