Update Sock contract to mirror Phoenix.Socket.Transport

[mtrudel]

As discussed at phoenixframework/phoenix#4973 and planned for in phoenixframework/phoenix#5003, this work updates the Sock API to mirror Phoenix.Socket.Transport, with a few additive changes:

• c:Sock.init/1 can now return the same set of tuples as c:Sock.handle_in/2 et al. This allows init to send messages at connection time
• c:Sock.terminate/1 defines more possible arguments, codifying a number of common termination reasons such as server shutdown or timeout.
• Any of the callbacks which return {:push, ...} or {:reply, ...} may indicate the transmission of any type of frame
• c:Phoenix.Socket.Transport.start_link/1 moves to c:Phoenix.Socket.start_link/1