net: ensure identical queries are not sent multiple times in builtin stub resolver
Prevents non-rooted queries with > ndots dots from being tried twice on error.
Fixes #8616.
Benchmark results on linux/amd64
benchmark old ns/op new ns/op delta
BenchmarkGoLookupIPNoSuchHost
8212394 4413293 -46.26%
benchmark old allocs new allocs delta
BenchmarkGoLookupIPNoSuchHost 216 108 -50.00%
benchmark old bytes new bytes delta
BenchmarkGoLookupIPNoSuchHost 17460 8726 -50.02%
LGTM=iant, mikioh.mikioh
R=golang-codereviews, iant, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/
137870043