Move Logging Code into Logging Guard in Learner

    void readPacket(QuorumPacket pp) throws IOException {
...
        long traceMask = ZooTrace.SERVER_PACKET_TRACE_MASK;
        if (pp.getType() == Leader.PING) {
            traceMask = ZooTrace.SERVER_PING_TRACE_MASK;
        }
        if (LOG.isTraceEnabled()) {
            ZooTrace.logQuorumPacket(LOG, traceMask, 'i', pp);
        }
    }