schedule: adjust store balance rate meaning

[rleungx]

What problem does this PR solve?

cherry-pick #1589.

Check List

Tests

• Unit test

Code changes

• Has configuration change

[codecov-io]

Codecov Report

Merging #1591 into release-3.0 will decrease coverage by 0.19%.
The diff coverage is 33.33%.

@@              Coverage Diff               @@
##           release-3.0    #1591     +/-   ##
==============================================
- Coverage        67.83%   67.63%   -0.2%     
==============================================
  Files              159      159             
  Lines            16258    16258             
==============================================
- Hits             11028    10996     -32     
- Misses            4273     4292     +19     
- Partials           957      970     +13

Impacted Files	Coverage Δ
server/schedule/mockcluster.go	82.25% <ø> (ø)	⬆️
server/config.go	82.33% <ø> (ø)	⬆️
server/api/store.go	51.87% <0%> (ø)	⬆️
server/schedule/operator_controller.go	86.38% <100%> (ø)	⬆️
pkg/testutil/operator_check.go	72.22% <0%> (-11.12%)	⬇️
pkg/etcdutil/etcdutil.go	86.04% <0%> (-9.31%)	⬇️
server/region_syncer/client.go	79.74% <0%> (-3.8%)	⬇️
server/leader.go	79.59% <0%> (-3.68%)	⬇️
server/util.go	75.49% <0%> (-2.65%)	⬇️
server/grpc_service.go	58.36% <0%> (-2.37%)	⬇️
... and 5 more

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[Connor1996]

LGTM