To prevent leaking connections, close any open Rows when the
context is cancelled. Also enforce context cancel while reading
rows off of the wire.

Change-Id: I62237ecdb7d250d6734f6ce3d2b0bcb16dc6fda7
Reviewed-on: https://go-review.googlesource.com/29957Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>