Prevent the SimpleSAML_Logger class from creating loops while trying to get...

Prevent the SimpleSAML_Logger class from creating loops while trying to get the track ID from the session. It must now be set manually by calling SimpleSAML_Logger::setTrackID(). Also allow SimpleSAML_Memcache to differentiate between a key not found in memcache and a request to memcache failed. If all servers are down, an exception is thrown and the user informed about the internal error. This hopefully resolves #264.