ospf: suppress delete using replacement

ospf: suppress delete using replacement

After a SPF run, OSPF deletes routes that have changed in terms of any
metric, type, and/or next-hops and re-adds them. Given that the Zebra-RIB
already support replacement semantics, we suppress deletes for routes
that will be added back again.

This has the following advantages. It reduces the number of IPC messages
between OSPF/Zebra. Also, in the current flow, a batch of route deletes
were followed by a batch of adds even for say a metric change.

With the change, routes are sent as "add" when they are modified. Zebra
already implicitly deletes older routes.

Signed-off-by: Ayan Banerjee <ayan@cumulusnetworks.com>
Reviewed-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Reviewed-by: Dinesh Dutt <ddutt@cumulusnetworks.com>
Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>