Scaffolding for event handling in future protocols

[johncantrell97]

Adds the concept of events to the library. Future protocols should:

• add Event enum variants
• give protocol message handler a reference to the pending_events EventQueue.
• push any relevant events as they are generated inside their protocol

End user is currently expected to poll for events using either:

• get_and_clear_pending_events in order to retrieve all currently queued events without blocking (returns empty vec if nothing present)

or

• get_next_event blocks until next event is ready and returns it

Perhaps we want an async version that returns a Future as well?

[tnull]

I also wonder if we need to persist this event queue? What happens when the node is shutdown and the users hasn't handled all events. Is the state of all protocols really so stateless that everything will start over? If not and we don't persist it, we need to make sure in each LSPS that every necessary event is re-generated on startup.

[tnull]

LGTM.