Zebra: Make redistribute do replace instead of del/add for better convergence

Zebra: Make redistribute do replace instead of del/add for better convergence

Ticket: CM-6768
Reviewed By: CCR-3207
Testing Done: bgpsmoke, smoke, topo to create failure

Redistributing routes goes through a del/add cycle whenever a redistributed
is updated. This del/add cycle causes disruption by causing traffic loss
for brief/long periods of time(6-8 s in case of OSPF). The modifications in
this patch remove the del/add cycle to ensure that this disruption doesn't
happen.

Also fixed sending no forwarding address when announcing IPv4 routes with IPv6
nexthops, and sending nexthop only when there is a single path.

Signed-off-by: Dinesh G Dutt <ddutt@cumulusnetworks.com>
Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Vivek Venkatraman <vivek@cumulusnetworks.com>