database/sql: close driver Stmt before releasing Conn

From the issue, which describes it as well as I could:

database/sql assumes that driver.Stmt.Close does not need the
connection.

see database/sql/sql.go:1308:

This puts the Rows' connection back into the idle pool, and
then calls the driver.Stmt.Close method of the Stmt it belongs
to.  In the postgresql driver implementation
(https://github.com/lib/pq), Stmt.Close communicates with the
server (on the connection that was just put back into the idle
pool).  Most of the time, this causes no problems, but if
another goroutine makes a query at the right (wrong?) time,
chaos results.

In any case, traffic is being sent on "free" connections
shortly after they are freed, leading to race conditions that
kill the driver code.

Fixes Issue 5283

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8633044