Enh: store message arrival time for use in sorting

Messages can be delayed in transit for various reasons; in particular messages that are moderated may incur a delay of several days.

It might be useful to show when a message actually arrived.

It would also be useful to be able to display messages in arrival order: at present they are displayed in order by Date, which means that it is quite difficult to see which messages have arrived since one last looked at a list.

For messages that were archived, the timestamp on the From_ line could potentially be used.
However that would not work for imports.

It might be useful to add a new field to the database with a timestamp showing when it was added.
Also whether it was imported or not.

Start by reviewing the issue's proposal for storing message arrival timestamps and import status, then trace how messages are added, archived, imported, and sorted by Date. Done would mean an agreed design and implementation that records arrival time for all relevant messages and supports arrival-order display, including imports.

Written by the indexing model from the issue text.