Zebra: Eliminate unnecessary del-add upon static route addition

Zebra: Eliminate unnecessary del-add upon static route addition

When static routes are added, they get processed and potentially installed
in the RIB once. Subsequently, NHT is invoked and ends up scheduling the
route for processing again because this is the first time the nexthop is
resolved for NHT. This used to result in a del-add earlier (as noted in
the defect), but is a replace now. This change eliminates the unnecessary
replace by ensuring NHT is invoked first if the static route has a nexthop
that will be tracked by NHT.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Dinesh Dutt <ddutt@cumulusnetworks.com>
Ticket: CM-4573
Reviewed By: CCR-3903
Testing Done: Manual and bgpsmoke

Note: Updates/improves static route NHT patch(es).