*: tiny update for the whitelist plugin

[tiancaiamao]

What problem does this PR solve?

The whitelist plugin need those changes.

What is changed and how it works?

• expose the domain.GetEtcdClient function
• use flush status to notify the update of whitelist
• domainMap.Get now returns the domain if it's the only one

Check List

Tests

• Manual test (add detailed scripts or steps below

[codecov-io]

Codecov Report

Merging #9271 into master will decrease coverage by 0.03%.
The diff coverage is 2.85%.

@@            Coverage Diff             @@
##           master    #9271      +/-   ##
==========================================
- Coverage   67.25%   67.22%   -0.04%     
==========================================
  Files         371      371              
  Lines       77234    77268      +34     
==========================================
- Hits        51947    51943       -4     
- Misses      20649    20683      +34     
- Partials     4638     4642       +4

Impacted Files	Coverage Δ
plugin/spi.go	100% <ø> (ø)	⬆️
domain/domain.go	40.31% <0%> (-0.15%)	⬇️
executor/simple.go	66.39% <0%> (-2.88%)	⬇️
server/server.go	49.3% <0%> (-3.51%)	⬇️
session/tidb.go	77.93% <25%> (-1.65%)	⬇️
ddl/delete_range.go	77.77% <0%> (-1.59%)	⬇️
util/filesort/filesort.go	75.54% <0%> (-0.95%)	⬇️
executor/join.go	77.86% <0%> (-0.53%)	⬇️
executor/index_lookup_join.go	77.91% <0%> (+0.63%)	⬆️
store/tikv/lock_resolver.go	42.65% <0%> (+0.94%)	⬆️

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[jackysp]

LGTM
PTAL @lysu

[jackysp]

/run-all-tests

[lysu]

LGTM