*: add config check mechanism

[rleungx]

What problem does this PR solve?

Since we use the config manager to manage the configuration of all components. PD also should support it. It can be reviewed after #2041 is merged.

What is changed and how it works?

This PR adds a new goroutine to register config to the config manager and get the update from the config manager to apply it locally.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #2049 into master will decrease coverage by 0.11%.
The diff coverage is 45.39%.

@@            Coverage Diff             @@
##           master    #2049      +/-   ##
==========================================
- Coverage      77%   76.89%   -0.12%     
==========================================
  Files         194      194              
  Lines       19572    19724     +152     
==========================================
+ Hits        15072    15166      +94     
- Misses       3366     3427      +61     
+ Partials     1134     1131       -3

Impacted Files	Coverage Δ
server/config/config.go	87.5% <ø> (ø)	⬆️
server/config/option.go	87.44% <42.85%> (-3.8%)	⬇️
server/server.go	75.51% <45.65%> (-8.01%)	⬇️
server/schedulers/shuffle_hot_region.go	64.58% <0%> (-5.21%)	⬇️
client/base_client.go	84.82% <0%> (-2.76%)	⬇️
client/config_client.go	54.54% <0%> (-2.03%)	⬇️
client/client.go	67.92% <0%> (-0.54%)	⬇️
server/member/leader.go	78.06% <0%> (-0.52%)	⬇️
... and 12 more

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[lhy1024]

LGTM

[NingLin-P]

LGTM

[rleungx]

/run-all-tests

[rleungx]

/merge

[sre-bot]

/run-all-tests