Update to RabbitMQ 3.4.0

Updated from the mercurial repository, tag "rabbitmq_v3_4_0".

Line 78 of `amqp_direct_connection.erl` was edited manually to underscore the Node variable name and remove the compiler warning `src/amqp_direct_connection.erl:78: Warning: variable 'Node' is unused`. I'll report this upstream in case it has not been fixed in master.

This update was tested using the test suite in the Elixir amqp wrapper.

Detail of the changed line:
``` erlang
handle_message({'DOWN', _MRef, process, _ConnSup, Reason},
               State = #state{node = _Node}) ->
    {stop, {remote_node_down, Reason}, State};
```