zebra: Limit meta_queue insertion to one time.

zebra: Limit meta_queue insertion to one time.

Modify the meta_queue insertion such that we only enqueue
the route_node into one meta_queue instead of several.

Suppose we have multiple route_entries associated with
a particular node from rip, bgp, staticd.  If we receive a
route update from rip, we would enqueue the route_node into
the 1, 2, 3 meta-nodes.  Which means that we would run
the entire process of figuring out a route 3 times, while
nothing would change the second two times.

Modify the code to choose the lowest meta-queue and
install it into that one for processing.

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