Bug: does not handle resent messages

As per RFC 2822, an original message can be resent to the same mailing list.

This message must have the following headers prefixed to it:
Resent-From:
Resent-Date:
It should also have the following:
Resent-Message-ID:
It may have other headers added (see the RFC)

At present, these headers are not used.

Since the rest of the resent message is the same, failure to take account of the headers may mean that the original and new message are given the same database id; i.e. only one of them will be stored.

If the Resent-Message-ID: is present, that should perhaps be used instead of the original message-id when generating the database id. However the orginal M-Id needs to be kept for searches.

There is at least one example in the ASF archives, see: INFRA-15174

Start by reviewing RFC 2822 and the ASF example referenced as INFRA-15174, then trace the message-header parsing and database-ID generation paths. Determine how resent headers are currently handled; done means resent messages receive distinct stored IDs while the original Message-ID remains available for searches, with the behavior covered by a regression test.

Written by the indexing model from the issue text.