*: make hot region scheduler configurable

[nolouch]

Signed-off-by: nolouch nolouch@gmail.com

What problem does this PR solve?

In some situations, the hot region scheduling is too intense and affects the latency jitter.

What is changed and how it works?

let something in the hot region schedule be configurable.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #1412 into master will increase coverage by 0.1%.
The diff coverage is 90.9%.

@@            Coverage Diff            @@
##           master    #1412     +/-   ##
=========================================
+ Coverage   67.52%   67.62%   +0.1%     
=========================================
  Files         157      157             
  Lines       14774    14800     +26     
=========================================
+ Hits         9976    10009     +33     
+ Misses       3874     3872      -2     
+ Partials      924      919      -5

Impacted Files	Coverage Δ
server/schedule/basic_cluster.go	92.15% <ø> (-3.93%)	⬇️
server/config.go	82.18% <100%> (+0.39%)	⬆️
server/server.go	77.38% <100%> (-0.4%)	⬇️
server/option.go	91.25% <100%> (+2.78%)	⬆️
server/schedulers/hot_region.go	79.45% <71.42%> (-0.08%)	⬇️
server/cluster_info.go	87.47% <83.33%> (+0.05%)	⬆️
server/schedule/mockcluster.go	82.92% <90%> (+0.26%)	⬆️
server/schedulers/shuffle_hot_region.go	64.1% <0%> (-6.42%)	⬇️
server/util.go	71.12% <0%> (-2.82%)	⬇️
... and 9 more

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[rleungx]

The rest LGTM.

[huachaohuang]

Seems this is a good case for a Schrodinger test :)

[zhouqiang-cl]

/run-all-tests