Fix context query cancellation

Previous commits had a race condition due to not waiting for the PostgreSQL
server to close the cancel query connection. This made it possible for the
cancel request to impact a subsequent query on the same connection. This
commit sets a flag that a cancel request was made and blocks until the
PostgreSQL server closes the cancel connection.

query.go

@@ -419,6 +419,11 @@ func (c *Conn) QueryRow(sql string, args ...interface{}) *Row {
 }
 
 func (c *Conn) QueryContext(ctx context.Context, sql string, args ...interface{}) (rows *Rows, err error) {
+	err = c.waitForPreviousCancelQuery(ctx)
+	if err != nil {
+		return nil, err
+	}
+
 	c.lastActivityTime = time.Now()
 
 	rows = c.getRows(sql, args)