…o avoid CPU hit

In case ipv6 global addresses were configured on L3 interfaces and used for peering,
and routing protocol was using link-local addresses on the same interfaces as prefered nexthops,
the link-local addresses could be aged out after a while due to no activities towards the link-local
addresses themselves. And when we receive new routes with the link-local nexthops, SONiC won't insert
them to the HW, and thus cause looping or blackholing traffic.

Global ipv6 addresses on L3 interfaces between switches are refreshed by BGP keeplive and other messages.

On server facing side, traffic may hit fowarding plane only, and no refresh for the ipv6 neighbor entries regularly.
This could age-out the linux kernel ipv6 neighbor entries, and HW neighbor table entries could be removed,
and thus traffic going to those neighbors would hit CPU, and cause traffic drop and temperary CPU high load.

Also, if link-local addresses were not learned, we may not get them at all later.

It is intended to fix all above issues.

Changes:
Add ndisc6 package in swss docker and use it for ipv6 ndp ping to update the neighbors' state on Vlan interfaces
Change the default ipv6 neighbor reachable timer to 30mins
Add periodical ipv6 multicast ping to ff02::11 to get/refresh link-local neighbor info.

Add PORTCHANNEL_INTERFACE interface for ipv6 multicast ping
format issue