ospf6d: abr summary advertise best route

ospf6d: abr summary advertise best route

ABR summary should contain best intra prefix LSA.
There might be a case where intra-prefix route with different
cost from different advertising router present, summary
advertisement should only consider best intra-prefix route.

A route which is marked for remove, while purging the route's
ABR summary LSA also remove summary route from the summary
table.

Ticket:CM-22665
Testing Done:
Validate on ABR topology, intra-prefix route with
different cost, a higher cost intra-prefix route skip from sending
ABR summary LSA.

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>