zebra: Do not allow same rib_dest_t be queued multiple times to meta queue

zebra: Do not allow same rib_dest_t be queued multiple times to meta queue

If we have already scheduled a node to be on the meta_queue, there is no
need to schedule it up again.

On startup we are calling rib_update() multiple times per connected route.
Due to the multiple ways we can get callbacks for adding a connected route
I decided it was best to just improve meta_queue performance as opposed
to trying to figure out all the different ways across all the platforms
that we can decide that a connected route has changed.  This appears
to solve the issue with a very large # of interfaces coming up
at the same time on startup.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>