Log SMTP traces for failed deliveries from queue for easier debugging

[mjl-]

When deliveries of messages in the queue fail, we should store the SMTP protocol traces including timing information (useful for timeouts) with the message in the queue. When all delivery attempts have failed, we could include all these details in the DSN to the user, or perhaps just log the logging lines of the last failed delivery. This should help with debugging deliverability issues.

The traces should not contain any message data. The current "trace"-level logging should suffice, it only shows that data writes happen, not their content.

Based on question about timeouts during delivery from robSlgm.