EVPN-VXLAN: Core isolation detection

With the support for LACP on the customer edge-to-provider edge (CE-PE) link, at the time of core isolation (if connectivity toward all other EVPN-MPLS PE devices is lost), the CE-facing interface of the multihomed PE device is set to the standby state, thereby blocking data traffic transmission from and toward the multihomed CE device. After the core recovers from the failure, the interface state switches back from standby to active.