bgpd: bmp, define hook for route distinguisher updates

bgpd: bmp, define hook for route distinguisher updates

At startup, if bmp loc-rib is enabled, the peer_id of the
loc-rib per peer header message has the route distinguisher set to 0:0.
Actually, the route distinguisher has been updated after the peer up
message is sent, and the information is not refreshed.

Create a hook API to handle route distinguisher config events: pre and
post configuration. Use that hook in BMP module to send peer down, and
peer up events when necessary.

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