No need of separate function to filter duplicates.
Just merge ranges in-place
```
go test -v -run=xxx -bench=BenchmarkMergeOld -benchmem
BenchmarkMergeOld-8 100000 13524 ns/op 1104 B/op 8 allocs/op
go test -v -run=xxx -bench=BenchmarkMergeNew -benchmem
BenchmarkMergeNew-8 100000 13432 ns/op 936 B/op 3 allocs/op
```
Not much performance boost, but less memory allocation
and simpler
Test 15, counting from zero, in TestGetMergedPerms
in etcd/auth/range_perm_cache_test.go, was trying
incorrectly assert that [a, b) merged with [b, "")
should be [a, b). Added a test specifically for
this. This patch fixes the incorrect larger test
and the bugs in the code that it was hiding.
Fixes#6359
Currently the auth mechanism doesn't support permissions of range
request. It just checks exact matching of key names even for range
queries. This commit adds a mechanism for setting permission to range
queries. Range queries are allowed if a range of the query is [begin1,
end1) and the user has a permission of reading [begin2, range2) and
[begin1, end2) is a subset of [begin2, range2). Range delete requests
will follow the same rule.
Gyu-Ho Lee