Unified ISSU for VRRPv3: Avoid adjacency flap

Enhancements have been made to maintain protocol adjacency with peer routers during unified ISSU and to maintain interoperability among equipment and with other Junos OS releases and other Juniper Networks products. This design is for VRRPv3 only. VRRPv1 and VRRPv2 are not supported. The show vrrp command output is updated to display unified ISSU information.