Add context methods to sql and sql/driver methods. If
the driver doesn't implement context methods the connection
pool will still handle timeouts when a query fails to return
in time or when a connection is not available from the pool
in time.

There will be a follow-up CL that will add support for
context values that specify transaction levels and modes
that a driver can use.

Fixes #15123

Change-Id: Ia99f3957aa3f177b23044dd99d4ec217491a30a7
Reviewed-on: https://go-review.googlesource.com/29381Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>