Upstreaming L3VPN bugs to master

Issue-1: When VpnInterface has a secondary adjacency like invisible ip, if tunnel EP is deleted , VpnInterface Op DS is updated with empty nexthoplist. During this time , invisible ip goes missing from VpnInterface Op DS due to MDSAL.syncwrite() being done in updateVpnInterfaceOnTepDelete. Another fix being done here is , invisible ip FIB entry is updated with empty nexthop and route is withdrawn/advertized to BGP.

Issue-2: prefix-A part of subnet-A/network-A/VPN-A. The refcount for prefix-A is 1. When VPN-B is importing from VPN-A , refcount is 2. But when prefix-B part of VPN-B is deleted , refcount was not decremented. For self-imported routes , when the prefix is deleted , refcount is decremented.