Index node power state and power target events

[LeeSmet]

Right now, only the current power state and power target of a node are saved. It would be better to index all power state/target change events, to a historic log of these can be queried, similar to how uptime reports work. This way, consumers of the API can also verify that the farmer bot is both working properly, and account for it (to e.g. calculate uptime)

[sameh-farouk]

Update:
PR is ready for review.