Update WS server client to pass in topic context and connection

[benjreinhart]

Some improvements to ws client:

1. Pass more context into handler. This allows us to access params (like sessionId) from the topic name (e.g., sessions:123). Previously we would also pass this value in the payload but that is redundant since we should be able to get it from the topic. We also pass in the event in case that is ever helpful.
2. Pass in a "connection" to the handler. This allows us to "reply" to one connected client. We've been broadcasting messages to all clients. I'll be using this in some future work.
3. Remove "outgoing" validating on the server as it's not really needed and was creating more noise for no gain.