Add a web socket call concept for connecting.

Similar to HTTP and Call, the WebSocketCall is a representation of a pending HTTP request and subsequent upgrade to speak web sockets. Upon synchronous execution you are handed a WebSocket instance for synchronous writing and also pass in a WebSocketListener for async callbacks due to reading.

The API changes in this commits also generalize WebSocket such that it's agnostic to being a client or server peer.

Not implemented yet is an async version of the initial HTTP request (enqueue).

Closes #1173. Refs #193.
This matches `Call` for testing, I guess.