scheduler: use high priority for the hot region related operators

[rleungx]

What problem does this PR solve?

Hot region related operators should be the high priority.

What is changed and how it works?

This PR does the thing mentioned above.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #1492 into master will decrease coverage by 0.17%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master    #1492      +/-   ##
==========================================
- Coverage   67.44%   67.27%   -0.18%     
==========================================
  Files         158      158              
  Lines       15459    15465       +6     
==========================================
- Hits        10427    10404      -23     
- Misses       4088     4110      +22     
- Partials      944      951       +7

Impacted Files	Coverage Δ
server/config.go	82.35% <100%> (ø)	⬆️
server/schedulers/hot_region.go	76.83% <50%> (-0.61%)	⬇️
pkg/etcdutil/etcdutil.go	84.84% <0%> (-9.1%)	⬇️
server/etcd_kv.go	63.63% <0%> (-7.28%)	⬇️
server/schedulers/shuffle_hot_region.go	64.1% <0%> (-6.42%)	⬇️
server/api/redirector.go	78.94% <0%> (-5.27%)	⬇️
server/leader.go	78.57% <0%> (-4.21%)	⬇️
server/schedule/basic_cluster.go	88.67% <0%> (-3.78%)	⬇️
server/region_syncer/server.go	78.44% <0%> (-3.45%)	⬇️
server/util.go	70.27% <0%> (-2.71%)	⬇️
... and 7 more

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