A sequential lookup using any non-canceled context has a risk of
returning the result of the previous lookup for a canceled context (i.e.
an error).

This is already prevented for timed out context by forgetting the host
immediately and extending this to also compare the error to
`context.Canceled` resolves this issue.

Fixes #22724

Change-Id: I7aafa1459a0de4dc5c4332988fbea23cbf4dba07
Reviewed-on: https://go-review.googlesource.com/77670
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>