fix ping synchronizing bug


When I tried JSON API of VoltDB, it will report the error "JSON interface exception: No connections." after a while. Then the JSON API can not be used anymore but only shows the same error.

There are two problems needed to fix. First, `m_outstandingPing ` in `NodeConnection ` is used to update the ping status. It is needed to be synchronized. Handled pings would not be noticed by `CallExpiration` in original codes because the synchronizing problem of `m_outstandingPing`.

Second, the updating of `m_lastResponseTimeNanos` in `handleMessage` seems to be put in incorrect place. So although pings are handled in `handleMessage`, `m_lastResponseTimeNanos` is not updated correspondingly. It causes wrong disconnection of timeout in original codes.