Fix data race with Rows and ConnPool

In an effort to reduce memory allocations, Rows was stored on the
Conn. This caused a race condition where Rows are closed and this
returns the Conn to the Pool. The Pool could then give out the Conn
again. Rows would then be reanimated and the original Rows could reclose
it.

query.go

@@ -354,8 +354,7 @@ func (rows *Rows) Values() ([]interface{}, error) {
 // be returned in an error state. So it is allowed to ignore the error returned
 // from Query and handle it in *Rows.
 func (c *Conn) Query(sql string, args ...interface{}) (*Rows, error) {
-	c.rows = Rows{conn: c, startTime: time.Now(), sql: sql, args: args, logger: c.logger}
-	rows := &c.rows
+	rows := &Rows{conn: c, startTime: time.Now(), sql: sql, args: args, logger: c.logger}
 
 	ps, ok := c.preparedStatements[sql]
 	if !ok {