add last asserted block height to /hotspots json

[allenan]

in addition to the height the hotspot was added, it'd be useful to see the height it was last asserted, to get a sense of which hotspots in a geography have recently been asserted there

[madninja]

in addition to the height the hotspot was added, it'd be useful to see the height it was last asserted, to get a sense of which hotspots in a geography have recently been asserted there

so do you want this as a separate endpoint, or added to all hotspot json output? the latter is a bit more expensive