TryFrom<EnvelopeEntity> for Envelope contains strange (and outdated?) logic

[rubdos]

I came across this while implementing #255

match entity.source_uuid.as_deref() {
    Some(uuid) => {
        let address = uuid.try_into()?;
        Ok(Envelope::new_with_source(entity, address))
    },
    None => Ok(Envelope::new_from_entity(entity)),
}

I'm not sure why we don't handle the address in a single new constructor.

Additionally, the initialization in new_from_entity contains this funny part:

server_guid: entity.source_uuid,

Git blame comes to me, so I did that. Not sure why.

We should probably revisit the whole logic here and adopt it anew from Android/SD's logic.