VCCV for L2 circuit and L2VPN

This feature introduces Virtual Circuit Connectivity Verification (VCCV) for LDP-signaled Layer 2 circuits and BGP-signaled Layer 2 VPNs, with VCCV using Bidirectional Forwarding Detection (BFD). BFD provides a low-resource mechanism for the continuous monitoring of the pseudowire data path and for detecting data plane failures. This feature provides support for asynchronous mode BFD for VCCV as described in RFC 5885, Bidirectional Forwarding Detection (BFD) for the Pseudowire Virtual Circuit Connectivity Verification (VCCV).