placement: add manager and FitRegion interface

[disksing]

Signed-off-by: disksing i@disksing.com

What problem does this PR solve?

Continue merging placement rules.

What is changed and how it works?

• add RuleManager container
• add FitRegion interface.

Check List

Tests

• Unit test

Code changes

• Has persistent data change

[codecov-io]

Codecov Report

Merging #1865 into master will decrease coverage by 0.02%.
The diff coverage is 63.54%.

@@            Coverage Diff             @@
##           master    #1865      +/-   ##
==========================================
- Coverage   77.94%   77.91%   -0.03%     
==========================================
  Files         161      163       +2     
  Lines       16076    16279     +203     
==========================================
+ Hits        12530    12684     +154     
- Misses       2551     2596      +45     
- Partials      995      999       +4

Impacted Files	Coverage Δ
server/cluster.go	83.1% <0%> (-0.13%)	⬇️
pkg/mock/mockcluster/mockcluster.go	89.58% <0%> (-0.63%)	⬇️
server/schedule/placement/fit.go	0% <0%> (ø)
server/schedule/placement/rule_manager.go	85.43% <85.43%> (ø)
server/region_syncer/client.go	77.63% <0%> (-7.9%)	⬇️
server/schedulers/random_merge.go	58.13% <0%> (-4.66%)	⬇️
server/member/leader.go	75.51% <0%> (-3.07%)	⬇️
client/client.go	68.6% <0%> (-1.73%)	⬇️
server/grpc_service.go	58.62% <0%> (-1.08%)	⬇️
pkg/etcdutil/etcdutil.go	88.4% <0%> (ø)	⬆️
... and 11 more

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[Luffbee]

LGTM

[disksing]

/merge

[sre-bot]

/run-all-tests