database/sql: Add an optional Queryer-Interface (like Execer)

Completly the same like the Execer-Interface, just for Queries.
This allows Drivers to execute Queries without preparing them first

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7085056

Committer: Brad Fitzpatrick <bradfitz@golang.org>