portlist: add BenchmarkGetListIncremental

[bradfitz]

In contrast to BenchmarkGetList, this new BenchmarkGetListIncremental acts like what happens in practice, remembering the previous run and avoiding work that's already been done previously.

Currently:

BenchmarkGetList
BenchmarkGetList-8                           100          11011100 ns/op           68411 B/op       2211 allocs/op
BenchmarkGetList-8                           100          11443410 ns/op           69073 B/op       2223 allocs/op
BenchmarkGetList-8                           100          11217311 ns/op           68421 B/op       2197 allocs/op
BenchmarkGetList-8                           100          11035559 ns/op           68801 B/op       2220 allocs/op
BenchmarkGetList-8                           100          10921596 ns/op           69226 B/op       2225 allocs/op
BenchmarkGetListIncremental
BenchmarkGetListIncremental-8                168           7187217 ns/op            1192 B/op         28 allocs/op
BenchmarkGetListIncremental-8                172           7004525 ns/op            1194 B/op         28 allocs/op
BenchmarkGetListIncremental-8                162           7235889 ns/op            1221 B/op         29 allocs/op
BenchmarkGetListIncremental-8                164           7035671 ns/op            1219 B/op         29 allocs/op
BenchmarkGetListIncremental-8                174           7095448 ns/op            1114 B/op         27 allocs/op

Updates #5958