util/conn: close old connection before reconnect (#982)

util/conn/manager.go

@@ -117,6 +117,7 @@ func (m *Manager) loop() {
 		case err := <-m.putc:
 			if err != nil && conn != nil {
 				m.logger.Log("err", err)
+				conn.Close()
 				conn = nil                            // connection is bad
 				reconnectc = m.after(time.Nanosecond) // trigger immediately
 			}