IdealWifiManager not working if stations are moving

As stated by @link2xt, in a scenario where STA moves away from AP and then stays there, SNR reduces and never becomes higher. In this case, m_lastSnrObserved stays high and high MCS is always selected, so no frames are successfully transmitted anymore.

An identified solution would be to add a Reset function called in DoReportFinalRtsFailed and DoReportFinalDataFailed:

IdealWifiManager::Reset (WifiRemoteStation *station)
{
  NS_LOG_FUNCTION (this << station);
  IdealWifiRemoteStation *st = (IdealWifiRemoteStation *)station;
  st->m_lastSnrObserved = 0.0;
  st->m_lastSnrCached = CACHE_INITIAL_VALUE;
  st->m_lastMode = GetDefaultMode ();
  st->m_nss = 1;
}

A test case should be generated as well.