change unknown-unicast handling to flooding instead of proxy-arp

* delete router-utils
* change tests: using `trema send_packets' to generate phosts,
  sender phost does not generate arp and generate unicast ipv4 directly,
  and receiver phost does not reply ipv4 packet. (it reply arp request)
  so that, if without using proxy-arp in test,
  it need to send packets from each host to another one, bidirectionally.