TODO

all:
1, netns support.

nf_conntrack_colo:
1, create the colo extension only when conn is unconfirmed.
   right now, we only create colo extension on the direction
   which guest sending packet. at this time, the conn may be
   confirmed already, so we should add target on the --physdev-out
   too.

2, use the nfnetlink interface, get rid of colo netlink.

3, id order to remove the limitation of netlink portid. use
   idmap to replace the colo_nodes array.

PMYCOLO:
1, do not use nfqueue to queue packets of master guest, this will
   cause netlink nfqueue loss effective. we should reuse the nfqueue_entry
   and NF_STOLEN.

2, fix the already know kernel bridge bug, this will triger proxy painc if
   the client is the master host. http://patchwork.ozlabs.org/patch/412388/

3, maybe we can put the enqueue_[tcp|udp|icmp]_packet and compare_[tcp|udp|icmp]
   to the nf_conntrack_l4proto.