BFD: basic asynchronous session up/down

This is a work-in-progress basic BFD session handling. Only
asynchronous mode is supported at the moment. Setting the session flags
doesn't work.

Change-Id: Idba27f721b5c35be5a66a6d202a63d23ff7ecf6f
Signed-off-by: Klement Sekera <ksekera@cisco.com>

test/template_bd.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 
-from abc import abstractmethod
+from abc import abstractmethod, ABCMeta
 
 from scapy.layers.l2 import Ether, Raw
 from scapy.layers.inet import IP, UDP
@@ -8,6 +8,7 @@ from scapy.layers.inet import IP, UDP
 
 class BridgeDomain(object):
     """ Bridge domain abstraction """
+    __metaclass__ = ABCMeta
 
     @property
     def frame_pg0_to_pg1(self):