Add request lifetime methods to traffic plugins

Traffic plugins (e.g. monitor, logging, and relay) need to receive func calls at different stages of an incoming request. Update the `relay/plugins/traffic/TrafficPlugin` interface with those methods as well as a return value that indicates whether the request should be dropped, has been handled, etc.

We need to keep in mind that connections may be upgraded for WebSocket use, use one of the keep-alive tactics, or fail at any point.

This Issue is blocked by #1 and #2