config: improve scheduler config reload behavior (#2592)

[ti-srebot]

cherry-pick #2592 to release-4.0

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Signed-off-by: disksing i@disksing.com

What problem does this PR solve?

Stale scheduler may appear after PD leader change, caused by the reload behavior that it merges in-memory config with persist config.

What is changed and how it works?

Drop in-memory state and merge default schedulers with persist configuration.

Check List

Tests

• Unit test
• Manually test

start 3pd + 1tikv
add evict-leader-scheduler-1
remove evict-leader-scheduler-1
kill pd1
show schedulers
make sure there is no evict-leader-scheduler-1

Release note

• Fix the problem of stale scheduler after leader change

[codecov-commenter]

Codecov Report

Merging #2608 into release-4.0 will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-4.0    #2608      +/-   ##
===============================================
- Coverage        77.09%   77.01%   -0.08%     
===============================================
  Files              205      205              
  Lines            22138    22119      -19     
===============================================
- Hits             17068    17036      -32     
- Misses            3765     3779      +14     
+ Partials          1305     1304       -1     

Impacted Files	Coverage Δ
server/config/config.go	76.65% <100.00%> (-0.69%)	⬇️
server/config/persist_options.go	89.72% <100.00%> (-0.72%)	⬇️
server/schedulers/shuffle_hot_region.go	63.47% <0.00%> (-11.31%)	⬇️
pkg/etcdutil/etcdutil.go	82.60% <0.00%> (-8.70%)	⬇️
server/region_syncer/client.go	78.62% <0.00%> (-3.06%)	⬇️
server/member/leader.go	72.37% <0.00%> (-1.95%)	⬇️
server/server.go	75.86% <0.00%> (-0.76%)	⬇️
server/schedule/operator_controller.go	81.25% <0.00%> (-0.17%)	⬇️
server/cluster/cluster.go	80.99% <0.00%> (+0.31%)	⬆️
... and 8 more

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[HunDunDM]

LGTM

[HunDunDM]

/merge

[ti-srebot]

/run-all-tests