[WIP] add tidwall/btree support

[siddontang]

Just an experiment. I tried to use tidwall/btree instead of origin google's, you can see that the perf result is promising. we can consider it in the near future.

go test -run=^$ -bench ^BenchmarkSortedRegions_
goos: darwin
goarch: amd64
pkg: github.com/tikv/client-go/v2/internal/locate
cpu: VirtualApple @ 2.50GHz

BenchmarkSortedRegions_SearchByKey/SearchByKey/Original-8         	 6363214	       189.0 ns/op
BenchmarkSortedRegions_SearchByKey/SearchByKey/V2-8               	 9601129	       124.4 ns/op
BenchmarkSortedRegions_ReplaceOrInsert/ReplaceOrInsert/Original-8 	 5168029	       230.7 ns/op
BenchmarkSortedRegions_ReplaceOrInsert/ReplaceOrInsert/V2-8       	 5801372	       207.6 ns/op
BenchmarkSortedRegions_AscendGreaterOrEqual/AscendGreaterOrEqual/Original-8         	  280526	      4251 ns/op
BenchmarkSortedRegions_AscendGreaterOrEqual/AscendGreaterOrEqual/V2-8               	  277137	      4249 ns/op

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