Define events

[tiero]

Also, I'm not so convinced that subscribing for spent/unspent utxos in neutrinod is ideal for UX. I would need to make 2 subscription to be notified of my utxo pool this way. Instead, 2 topics that make more sense to me are "transactions" and "utxos" (which are the abstractions built on top of neutrino scanner tx reports in vv/ocean). The idea is to get notified about every tx or utxo of interest, with the payload carrying the spent/unspent and confirmed/unconfirmed status.

Originally posted by @altafan in #28 (review)