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?