zebra: Use a nhe context dataplane and rib metaq

zebra: Use a nhe context dataplane and rib metaq

We will use a nhe context for dataplane interaction with
nextho group hash entries.

New nhe's from the kernel will be put into a group array
if they are a group and queued on the rib metaq to be processed
later.

New nhe's sent to the kernel will be set on the dataplane context
with approprate ID's in the group array if needed.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>