rethink notifications

[frolic]

• add unique key so we can query them in explorer, refer to specific events, and run analytics over them
• add caller/sender fields so we know who/what initiated an action

  been thinking about how to do the custom hook thing, maybe it would be simpler to use interop to consume messages from the world itself, we could just add a "caller" field to all notification table events, that identifies who initiated the tx
  that way we can consume that log and identify who killed someone, who mined something, etc
  basically we improve notifications to include all the data a verifier contract might need

• consider an additional field for "related entities" so we get easier querying/filtering via explorer without decoding specific notifs
• change notification action to bytes32? #241

[linear]

DUST-33 rethink notifications