Deal with client and server using different API versions

[armsnyder]

Find some way to deal with the client being out of date from the server. The client sends its version number to the server on "hello" action already. Maybe the server can send back an "updateRequired" action if the client is old? What about when/if there are different types of clients like mobile or web? All that really matters is the version of the messaging API that the client is using. Should we version the API separately from the client? What should the versioning scheme be? Should the server support multiple API versions simultaneously? What do websocket API versions even look like?