Add graceful exit contract to transports (#2107)

ref: phoenixframework/phoenix_pubsub#68

Previously a channel could gracefully terminate
using the stop semantics of a regular genserver;
however when restarting an application for deploys
the shutdown of the transport and channel processes
would be indistinguisable from an intentional channel
shutdown and would cause clients to incorrectly not
reconnect after server restart. This commit adds a
{:graceful_exit, channel_pid, %Phoenix.Socket.Message{}}
contract to distinguish an intentional channel exit from
what should be regarded as an error condition on the client.

test/phoenix/transports/transport_test.exs

@@ -28,12 +28,6 @@ defmodule Phoenix.Transports.TransportTest do
   ## on_exit_message
 
   test "on_exit_message/3" do
-    assert Transport.on_exit_message("foo", "1", :normal) ==
-           %Message{ref: "1", event: "phx_close", payload: %{}, topic: "foo"}
-    assert Transport.on_exit_message("foo", "1", :shutdown) ==
-           %Message{ref: "1", event: "phx_close", payload: %{}, topic: "foo"}
-    assert Transport.on_exit_message("foo", "1", {:shutdown, :whatever}) ==
-           %Message{ref: "1", event: "phx_close", payload: %{}, topic: "foo"}
     assert Transport.on_exit_message("foo", "1", :oops) ==
            %Message{ref: "1", event: "phx_error", payload: %{}, topic: "foo"}
   end