The gaol why we need to determine what ID should be used as stanza-id,
is so we can use the stanza-id in the future for deduplication.
Case we are the sender:
Either we look for a origin-id element, which we will include in the
Future in all messages we send, or until then the ID we set on the
message as attr.
Case we are the receiver:
If our server supports mam:2 we take the ID of the result element,
because mam:2 injects the archive ID live into every message we
receive. If we dont have mam:2 we fall back to the ID of the message
attr.
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