schedule: fix store maybe always overloaded (#1586)

[rleungx]

What problem does this PR solve?

cherry-pick #1586.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #1590 into release-3.0 will increase coverage by 0.11%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           release-3.0    #1590      +/-   ##
===============================================
+ Coverage        67.61%   67.73%   +0.11%     
===============================================
  Files              159      159              
  Lines            16275    16258      -17     
===============================================
+ Hits             11004    11012       +8     
+ Misses            4305     4281      -24     
+ Partials           966      965       -1

Impacted Files	Coverage Δ
server/schedule/scheduler.go	66.66% <ø> (ø)	⬆️
server/core/store.go	80.75% <100%> (+2.18%)	⬆️
server/cluster_info.go	86.73% <100%> (+1.51%)	⬆️
server/schedule/operator_controller.go	86.38% <100%> (+1%)	⬆️
server/core/store_option.go	100% <100%> (+5.88%)	⬆️
server/schedule/basic_cluster.go	89.09% <100%> (+3.12%)	⬆️
pkg/etcdutil/etcdutil.go	86.04% <0%> (-9.31%)	⬇️
server/etcd_kv.go	65.95% <0%> (-4.26%)	⬇️
server/namespace_cluster.go	81.81% <0%> (-3.9%)	⬇️
server/region_syncer/client.go	79.74% <0%> (-3.8%)	⬇️
... and 14 more

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