Link a channel with its writer when a direct connection is used

When a network connection is used, the channel and its associated writer are
linked because they are supervised by the same supervisor. So if the writer
dies, the channel dies too, because of the one_for_all strategie of the
supervisor.

For a direct connection, a channel and its writer have not the same
supervisor. So, an explicit link should be done between them. But, since the
commit f88e8c8, this link is not done anymore (In fact, the writer is linked
with the amqp_channel_sup_sup process).
Detect connection failure when a direct connection is used