hdr_cmn::DOWN or hdr_cmn::UP

Have to explicitly set direction of new pkts to DOWN before sending them down to the LL.
why: A variable direction_ in hdr_cmn is now used. This is used in the lower layers like LL, mac, phy etc to determine the
direction of the pkt flow. All incoming pkts are marked as UP by channel, which should be remarked as DOWN by agents
before sending them out into the network again.