[Upgrade notice] IPsec-VPN connections support the dual-tunnel mode

If an IPsec-VPN connection is associated with a transit router and the IPsec-connection has only one encrypted tunnel, the IPsec-VPN connection is interrupted when the tunnel is down. The dual-tunnel mode is introduced to improve the availability of IPsec-VPN connections. Each IPsec-VPN connection consists of two tunnels that serve as Equal-Cost Multipath Routing (ECMP) paths. When a tunnel is down, traffic is forwarded through the other tunnel. In a region that contains multiple zones, the two tunnels of an IPsec-VPN connection are automatically spread in different zones to implement zone-disaster recovery.