The current resolver uses a global lookupGroup which merges LookupIPAddr
calls together for lookups for the same hostname if used concurrently.
As a result only one of the resolvers is actually used to perform the
DNS lookup but the result is shared by all the resolvers.

This commit limits the scope of the lookupGroup to the resolver itself
allowing each resolver to make its own requests without sharing the
result with other resolvers.

Fixes #22908

Change-Id: Ibba896eebb05e59f18ce4132564ea1f2b4b6c6d9
Reviewed-on: https://go-review.googlesource.com/80775
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>