bgp: dynamic capabilities

[taspelund]

An implementation of BGP Dynamic Capabilities would enable the re-negotiation of BGP Capabilities without requiring the session to be reset.

This is especially useful when enabling/disabling address-families (e.g. IPv6 Unicast) without causing service disruption for existing IPv4 -- assuming our peers also support this capability.

FRR has implemented this draft but I'm not sure how widely deployed this is by other vendors.
GR is another mechanism that aims to allow these kind of changes to be similarly hitless (and is more widely deployed), but IMO is more complex.

Despite the above points, I still think this would be useful to have in the maghemite BGP implementation but I would say this is not a high priority request.