net: change type of dnsConfig.timeout from int to time.Duration

net: change type of dnsConfig.timeout from int to time.Duration

Instead of keeping the desired number of seconds and converting to
time.Duration for every query, convert to time.Duration when
building the config.

Updates #15473

Change-Id: Ib24c050b593b3109011e359f4ed837a3fb45dc65
Reviewed-on: https://go-review.googlesource.com/22548
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>