Enh: option to use timestamp from headers if there is no Date: header

Some messages don't have a Date: header.

The importer generally runs a long while after the messages were sent, so using the current time causes problems.

It would be useful to have an option for the importer to use another date, for example the From_ line normally has a timestamp. There are also timestamps in the Received: lines. The last one will normally be closest to the creation date of the mail.

The archiver may also benefit from the option. The mail will normally be received by the archiver soon after it is sent. But if there are multiple archivers, their timestamps may vary.

Trace the importer and archiver entry points that choose a message date, then inspect how the Date:, From_ and Received: headers are parsed. Define the option and its fallback order, and verify that messages without Date: use the selected header timestamp rather than the current time in both paths.

Written by the indexing model from the issue text.