bgp state machine history endpoint

[rcgoodfellow]

When you walk up to mgd and see that the established session duration is not what you expect, the next obvious question is why. You can go look through the logs and the BGP message history, but those are both very noisy channels. It would be nice to be able to do something like mgadm bgp fsm history <peer> to see the last N state transitions and the reasons for those transitions.