net: use dialTCP cancelation for DualStack dialing.

net: use dialTCP cancelation for DualStack dialing.

The previous Happy Eyeballs implementation would intentionally leak
connections, because dialTCP could not be reliably terminated upon
losing the race.

Now that dialTCP supports cancelation (plan9 excluded), dialParallel can
wait for responses from both the primary and fallback racers, strictly
before returning control to the caller.

In dial_test.go, we no longer need Sleep to avoid leaks.
Also, fix a typo in the Benchmark IPv4 address.

Updates #11225
Fixes #14279

Change-Id: Ibf3fe5c7ac2f7a438c1ab2cdb57032beb8bc27b5
Reviewed-on: https://go-review.googlesource.com/19390
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>