o router is implemented as a namespace, similar to test_brb.
  o using linux bridge to provide bridge functionality.
    In test_brb, a simple bpf program implemented limited
    bridge functionality.
  o on my local box, test_brb performance is roughly 12% better
    than test_brb2.
Signed-off-by: Yonghong Song <yhs@plumgrid.com>