Improve the events design

[johny-b]

This is a wide topic that should spawn several tasks.

Now we have events NewResource, ResourceClosed, ResorceDataChanged and never-emitted ResourceChangePossible.
This is not enough. But: it's super important to add only really useful events now, not "all events that maybe-one-day-could-have-some-use-and-we-thought-of-them-today".

Changes that seem reasonable to me now:

• Remove ResourceChangePossible. This will be super-complex to implement and doesn't seem very useful either way. Go back to the concept if we ever decide we need it.

• Add events ShutdownStarted, ShutdownFinished - this will make debugging easier (e.g. check if there was a full clean shutdown, check what happened during the shutdown and what happened just before etc). Consider also GolemStarted event, but I'm not sure about this one.

• Add BatchFinished event. Or, as an alternative: maybe emit ResourceClosed(batch)? But this might be confusing.

• Consider splitting NewResource to ResourceCreated and ResourceFound ("we created a new Resource object that corresponds to a Golem entity that was created before", e.g. when we first time interact with an Allocation created in a previous run). Consider following scenario:

  1. We register an event callback that is supposed to run whenever we create a new activity
  2. We create an Activity, callback is executed
  3. We restart the app
  4. We get a DebitNote for this activity and the callback executes again, because Activity object was created (but we didn't create an activity, GolemNode was told about a previously created activity)

  We might consider event nesting here - ResourceCreated and ResourceFound inheriting from NewResource.