Add checkpoint and pulse block heights.

[ianmacd]

It would be good if a node's detailed information screen also showed the node's recent checkpoints and pulses, as displayed by oxend print_sn_status:

    Checkpoints [Height,Voted]: [900709,yes], [901149,yes], [901173,yes], [901453,yes]
    Pulse [Height,Voted]: [900584,yes], [900824,yes], [901034,yes], [901173,yes], [901309,yes], [901406,yes], [901628,yes]