schedule, schedulers: always merge left into right

[overvenus]

Cherry picks #1564

Check List

Tests

• Unit test

Code changes

• Has configuration change

[Connor1996]

LGTM

[codecov-io]

Codecov Report

Merging #1583 into release-3.0 will increase coverage by 0.1%.
The diff coverage is 61.11%.

@@              Coverage Diff               @@
##           release-3.0    #1583     +/-   ##
==============================================
+ Coverage        67.41%   67.51%   +0.1%     
==============================================
  Files              159      159             
  Lines            16239    16253     +14     
==============================================
+ Hits             10947    10974     +27     
+ Misses            4317     4309      -8     
+ Partials           975      970      -5

Impacted Files	Coverage Δ
server/config.go	82.18% <ø> (ø)	⬆️
server/cluster_info.go	85.16% <0%> (-0.35%)	⬇️
server/option.go	89.92% <0%> (-0.71%)	⬇️
server/schedule/mockcluster.go	83.09% <100%> (+0.12%)	⬆️
server/schedulers/random_merge.go	61.53% <50%> (-5.13%)	⬇️
server/schedule/merge_checker.go	78.68% <77.77%> (+4.61%)	⬆️
server/grpc_service.go	59.22% <0%> (-0.86%)	⬇️
server/handler.go	53.14% <0%> (-0.49%)	⬇️
client/client.go	68.19% <0%> (+0.41%)	⬆️
server/schedule/region_scatterer.go	86.95% <0%> (+0.72%)	⬆️
... and 6 more

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