Fix Sock handlers to return a state

Currently they don't, and they should be closer to the return values used by Elixir's WebSock.ex:

@type handle_result() ::
  {:push, messages(), state()}
  | {:reply, term(), messages(), state()}
  | {:ok, state()}
  | {:stop, {:shutdown, :restart} | term(), state()}
  | {:stop, term(), close_detail(), state()}
  | {:stop, term(), close_detail(), messages(), state()}

Implementation notes

We can start with just push, ok, and stop.