Clean up eventEmitter stuff

[acolytec3]

Thinking through our eventEmitter garbage. We have at least 3 distinct eventEmitters inside of portalnetwork and we should really consider either putting these all on a central event bus or at least making them all StrictEventEmitters. They are:

1. portalnetwork class - this is a StrictEventEmitter with typed events (including an unused ContentAdded event
2. protocol base class - each subprotocol is a bare eventEmitter that currently only emits a ContentAdded event (used widely for gossip and resolving sendFindContent calls among other things
3. uTP looks like it has multiple event emitters inside it. I'm less concerned about these as they are contained within uTP which the rest of the portal network class should treat as a black box (not entirely successfully but still, it's a worthy goal)

So, at a minimum, let's do the following:

1. Remove the ContentAdded event from the IPortalNetworkEvents interface
2. Switch the protocol classes's EventEmitter to the StrictEventEmitter type and create an interface for the ContentAdded event
3. Make the uTP event emitters StrictEventEmitter with typed events.