This patch renames a number of types and functions making
the API more consistent.
* `Server` -> `Connection`
* `Client` -> `SendRequest`
* `Respond` -> `SendResponse`.
It also moves the handshake fns off of `Connection` and make
them free fns in the module. And `Connection::builder` is removed
in favor of `Builder::new`.
The Connection type is a `Future` that drives all of the IO of the
client connection.
The Client type is separate, and is used to send requests into the
connection.
