Query and Exec functions on DB first attempt to get a cached
connection before requesting the connection pool to ignore
the cache and get a new connection. This change aligns Stmt to
that behavior as well.

Fixes #20433

Change-Id: Idda5f61927289d7ad0882effa3a50ffc9efd88e6
Reviewed-on: https://go-review.googlesource.com/43790
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>