Direct connection reports server shutdown as error

[gotthardp]

amqp_direct_connection.handle_message({'DOWN', ...}, State) does not distinguish between the various reasons. As a consequence, the normal shutdown (rabbitmqctl stop) does generate an error report in the log file. (Can be reproduced by enabling the rabbitmq-metronome and setting log_levels={connection, debug}).

It think it would be better to handle the normal shutdown as a special case and not emit the error report.