statistics: use TopN

[Luffbee]

What problem does this PR solve?

Close issue #1997 .

What is changed and how it works?

Replace TwoQueueCache with TopN.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #2009 into master will decrease coverage by 0.39%.
The diff coverage is 90%.

@@            Coverage Diff            @@
##           master    #2009     +/-   ##
=========================================
- Coverage   77.77%   77.38%   -0.4%     
=========================================
  Files         180      180             
  Lines       18154    18152      -2     
=========================================
- Hits        14120    14047     -73     
- Misses       2972     3031     +59     
- Partials     1062     1074     +12

Impacted Files	Coverage Δ
server/statistics/hot_peer.go	100% <100%> (ø)	⬆️
server/statistics/hot_peer_cache.go	88.53% <86.66%> (-1.1%)	⬇️
pkg/cache/cache.go	0% <0%> (-62.17%)	⬇️
pkg/cache/two_queue.go	66.66% <0%> (-11.12%)	⬇️
pkg/etcdutil/etcdutil.go	82.6% <0%> (-8.7%)	⬇️
server/schedulers/base_scheduler.go	60.86% <0%> (-8.7%)	⬇️
server/region_syncer/client.go	78.94% <0%> (-6.58%)	⬇️
server/schedulers/shuffle_hot_region.go	61.29% <0%> (-5.38%)	⬇️
server/tso/tso.go	77.37% <0%> (-2.92%)	⬇️
server/statistics/store.go	71.15% <0%> (-2.89%)	⬇️
... and 11 more

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[lhy1024]

the rest LGTM

[nolouch]

/merge

[sre-bot]

/run-all-tests

[sre-bot]

@Luffbee merge failed.

[Luffbee]

/merge

[sre-bot]

/run-all-tests