server: use an individual package for leader

[rleungx]

What problem does this PR solve?

Part of modularizing PD. It should be reviewed after #1662 is merged.

What is changed and how it works?

This PR uses an individual package for leader.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #1668 into master will increase coverage by 0.13%.
The diff coverage is 82.38%.

@@            Coverage Diff             @@
##           master    #1668      +/-   ##
==========================================
+ Coverage   76.52%   76.66%   +0.13%     
==========================================
  Files         157      157              
  Lines       15352    15392      +40     
==========================================
+ Hits        11748    11800      +52     
+ Misses       2587     2579       -8     
+ Partials     1017     1013       -4

Impacted Files	Coverage Δ
server/util.go	65.67% <ø> (ø)	⬆️
server/api/redirector.go	78.94% <100%> (ø)	⬆️
pkg/testutil/testutil.go	93.75% <100%> (+8.03%)	⬆️
tests/cluster.go	85.99% <100%> (-0.34%)	⬇️
server/testutil.go	91.3% <100%> (-0.37%)	⬇️
server/grpc_service.go	59.74% <75%> (+0.64%)	⬆️
server/member/leader.go	80.09% <80.09%> (ø)
server/server.go	82.31% <83.47%> (+1.12%)	⬆️
server/api/member.go	61.6% <85.71%> (ø)	⬆️
pkg/metricutil/metricutil.go	90.62% <0%> (-9.38%)	⬇️
... and 11 more

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[shafreeck]

LGTM

[sre-bot]

/run-all-tests