enhance: simplify compaction tasks to reduce their memory overhead

[tedxu]

See #39080

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 81.16%. Comparing base (fd84ed8) to head (c67ba78).
Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/datacoord/compaction_task_mix.go	76.47%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #39121       +/-   ##
===========================================
+ Coverage   69.71%   81.16%   +11.44%     
===========================================
  Files         299     1404     +1105     
  Lines       26768   197668   +170900     
===========================================
+ Hits        18661   160434   +141773     
- Misses       8107    31614    +23507     
- Partials        0     5620     +5620     

Components	Coverage Δ
Client	79.53% <ø> (∅)
Core	69.71% <ø> (ø)
Go	83.09% <83.33%> (∅)

Files with missing lines	Coverage Δ
internal/datacoord/compaction.go	71.27% <ø> (ø)
internal/datacoord/compaction_task.go	100.00% <ø> (ø)
internal/datacoord/compaction_task_clustering.go	79.89% <100.00%> (ø)
internal/datacoord/compaction_task_l0.go	92.30% <100.00%> (ø)
internal/datacoord/compaction_task_mix.go	66.93% <76.47%> (ø)

... and 1100 files with indirect coverage changes

[XuanYang-cn]

/lgtm

[mergify]

@tedxu E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[XuanYang-cn]

/lgtm

[sre-ci-robot]

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