bfd: Implement Bidirectional Forwarding Detection.

bfd: Implement Bidirectional Forwarding Detection.

Traditionally, Open vSwitch has used a variant of 802.1ag "CFM" for
interface liveness detection.  This has served us well until now,
but has several serious drawbacks which have steadily become more
inconvenient.  First, the 802.1ag standard does not implement
several useful features forcing us to (optionally) break
compatibility.  Second, 802.1.ag is not particularly popular
outside of carrier grade networking equipment.  Third, 802.1ag is
simply quite awkward.

In an effort to solve the aforementioned problems, this patch
implements BFD which is ubiquitous, well designed, straight
forward, and implements required features in a standard way.  The
initial cut of the protocol focuses on getting the basics of the
specification correct, leaving performance optimizations, and
advanced features as future work.  The protocol should be
considered experimental pending future testing.

Signed-off-by: Ethan Jackson <ethan@nicira.com>