Fix message sender shutdown order

[annatisch]

The current messagesender_close function starts by clearing any pending messages by calling on_message_send_complete with an error state, and then attempts to detach the link.

However, if the link detach should fail for any reason (say, the connection is in an error state), then the link will also attempt to remove it's pending deliveries (on_session_state_change -> remove_all_pending_deliveries) which also calls the on_message_send_complete callback a second time.

This has the potential to fail in two ways - first, the on_message_send_complete callback must be idempotent (which the amqp_management on_message_send_complete is not) and secondly, the message has already been freed by the time the link attempts to settle it.

By moving the indicate_all_messages_as_error call to after the attempted detach, if the link fails to detach it will settle any pending messages and remove them from the message sender, thereby eliminating the double callback.