Zebra: Implement route replace for IPv6

Zebra: Implement route replace for IPv6

Zebra currently performs a delete followed by add when a route needs to be
modified. Change this to use the replace semantics of netlink so that the
operation can possibly be atomic.

Note: This patch handles IPv6 routes, IPv4 already performs a replace.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Dinesh Dutt <ddutt@cumulusnetworks.com>
Ticket: CM-5597
Reviewed By: CCR-3407
Testing Done: Manual testing of various scearnios (Vivek, Satish)

Note: This is an import of patch zebra-ipv6-route-replace.patch from 2.5-br.