This new method sends a PING frame with random payload to the peer and
wait for a PING ack with the same payload.

In order to support cancellation and deadling, the Ping method takes a
context as argument.

Fixes golang/go#15475

Change-Id: I340133a67717af89556837cc531a885d116eba59
Reviewed-on: https://go-review.googlesource.com/29965Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>