make the special client xid constant

if (replyHdr.getXid() == -2) {
    // -2 is the xid for pings
    if (LOG.isDebugEnabled()) {
        LOG.debug("Got ping response for sessionid: 0x"
                  + Long.toHexString(sessionId)
                  + " after "
                  + ((System.nanoTime() - lastPingSentNs) / 1000000)
                  + "ms");
    }
    return;
}
if (replyHdr.getXid() == -4) {
    // -4 is the xid for AuthPacket
    if (replyHdr.getErr() == KeeperException.Code.AUTHFAILED.intValue()) {
        state = States.AUTH_FAILED;
        eventThread.queueEvent(new WatchedEvent(Watcher.Event.EventType.None, Watcher.Event.KeeperState.AuthFailed, null));
        eventThread.queueEventOfDeath();
    }
    if (LOG.isDebugEnabled()) {
        LOG.debug("Got auth sessionid:0x" + Long.toHexString(sessionId));
    }
    return;
}
if (replyHdr.getXid() == -1) {