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>

vlib-api/vlibapi/api_helper_macros.h

@@ -197,7 +197,8 @@ _(to_netconf_server)                            \
 _(from_netconf_server)                          \
 _(to_netconf_client)                            \
 _(from_netconf_client)                          \
-_(oam_events)
+_(oam_events)                                   \
+_(bfd_events)
 
 /* WARNING: replicated in vpp/stats.h */
 typedef struct