statistics: introduce TopN

[Luffbee]

What problem does this PR solve?

First step of issue #1997 .

What is changed and how it works?

Add TopN, a container maintaining top-N items with TTL-based GC.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #1998 into master will increase coverage by 0.18%.
The diff coverage is 99.4%.

@@            Coverage Diff             @@
##           master    #1998      +/-   ##
==========================================
+ Coverage   77.56%   77.74%   +0.18%     
==========================================
  Files         178      179       +1     
  Lines       18240    18407     +167     
==========================================
+ Hits        14147    14310     +163     
- Misses       3013     3015       +2     
- Partials     1080     1082       +2

Impacted Files	Coverage Δ
server/statistics/topn.go	100% <100%> (ø)
server/statistics/util.go	96.15% <75%> (-3.85%)	⬇️
server/kv/etcd_kv.go	75.32% <0%> (-9.1%)	⬇️
server/schedulers/random_merge.go	61.81% <0%> (-3.64%)	⬇️
client/client.go	71.28% <0%> (-0.83%)	⬇️
server/grpc_service.go	58.53% <0%> (-0.44%)	⬇️
server/handler.go	52.83% <0%> (+0.47%)	⬆️
server/member/leader.go	78.57% <0%> (+0.51%)	⬆️
server/region_syncer/client.go	82.89% <0%> (+3.94%)	⬆️
... and 2 more

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[disksing]

the rest LGTM.

[sre-bot]

/run-all-tests