bgpd: add mpls vpn nh label bind cache struct and apis

bgpd: add mpls vpn nh label bind cache struct and apis

In the context of the ASBR facing an EBGP neighbor, or
facing an IBGP neighbor where the BGP updates received
are re-advertised with a modified next-hop, a new local
label will be re-advertised too, to replace the original
one.

Create a binding table, in the form of a hash list, from the
original labels to the new labels. Since labels can be the
same on several routers, set the next-hop and the label as
the keys. Add the needed API functions to manage the hash
list.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>