Improve search task merge policy

[yah01]

Signed-off-by: yah01 yang.cen@zilliz.com
/kind improvement
fix #22005

[codecov]

Codecov Report

Merging #22006 (4400614) into master (ffdb652) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #22006      +/-   ##
==========================================
+ Coverage   82.25%   82.32%   +0.06%     
==========================================
  Files         698      698              
  Lines       95343    95346       +3     
==========================================
+ Hits        78427    78492      +65     
+ Misses      13956    13891      -65     
- Partials     2960     2963       +3     

Impacted Files	Coverage Δ
internal/querynode/task_search.go	35.89% <100.00%> (+6.12%)	⬆️
internal/datacoord/segment_manager.go	86.98% <0.00%> (-2.22%)	⬇️
internal/datacoord/server.go	71.77% <0.00%> (-1.76%)	⬇️
internal/querynode/shard_cluster.go	95.88% <0.00%> (+0.39%)	⬆️
internal/datacoord/session_manager.go	83.75% <0.00%> (+0.50%)	⬆️
internal/distributed/datacoord/client/client.go	74.36% <0.00%> (+0.72%)	⬆️
internal/mq/msgstream/mq_msgstream.go	79.31% <0.00%> (+0.82%)	⬆️
internal/distributed/querycoord/client/client.go	80.37% <0.00%> (+1.06%)	⬆️
internal/rootcoord/quota_center.go	86.86% <0.00%> (+1.27%)	⬆️
internal/querycoordv2/session/cluster.go	70.04% <0.00%> (+1.44%)	⬆️
... and 7 more

[congqixia]

/cc @czs007

[czs007]

/approve
/lgtm

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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