fix: [2.4] Fix checkGeneralCapacity slowly

[bigsheeper]

Cache the general count to speed up checkGeneralCapacity.

issue: #37630

pr: #37976

[czs007]

/approve

[czs007]

/approve
/lgtm

[mergify]

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

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.25%. Comparing base (bd08b3b) to head (a53b330).
Report is 6 commits behind head on 2.4.

Files with missing lines	Patch %	Lines
internal/rootcoord/meta_table.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              2.4   #38074       +/-   ##
===========================================
+ Coverage   64.28%   72.25%    +7.97%     
===========================================
  Files         268     1074      +806     
  Lines       26479   168107   +141628     
===========================================
+ Hits        17021   121467   +104446     
- Misses       9458    42198    +32740     
- Partials        0     4442     +4442     

Files with missing lines	Coverage Δ
internal/rootcoord/constrant.go	100.00% <100.00%> (ø)
internal/rootcoord/create_collection_task.go	94.40% <100.00%> (ø)
internal/rootcoord/create_partition_task.go	100.00% <100.00%> (ø)
internal/rootcoord/meta_table.go	84.04% <92.30%> (ø)

... and 1001 files with indirect coverage changes

[czs007]

/approve
/lgtm

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bigsheeper, czs007

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