bgpd: graceful restart for vpnv4 address family

bgpd: graceful restart for vpnv4 address family

This patch enable the support of graceful restart for routes sets with
vpnv4 address family format. In this specific case, data model is
slightly different and some additional processing must be done when
accessing bgp tables and nodes.
The clearing stale algorithm takes into account the specificity where
the 2 node level for MPLS has to be reached.

Signed-off-by: Julien Courtat <julien.courtat@6wind.com>
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>