net: preserve unexpired context values for LookupIPAddr

To avoid any cancelation of the parent context from affecting
lookupGroup operations, Resolver.LookupIPAddr previously used
an entirely new context created from context.Background().
However, this meant that all the values in the parent context
with which LookupIPAddr was invoked were dropped.

This change provides a custom context implementation
that only preserves values of the parent context by composing
context.Background() and the parent context. It only falls back
to the parent context to perform value lookups if the parent
context has not yet expired.
This context is never canceled, and has no deadlines.

Fixes #28600

Change-Id: If2f570caa26c65bad638b7102c35c79d5e429fea
Reviewed-on: https://go-review.googlesource.com/c/148698


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>