webxdc.selfAddr not consistent for draft messages vs sent message

[adbenitez]

when the app is in draft mode the selfAddr is always the same:

e6edc6ba5a72cef8889657b0a5212318c5c06bc2647392cf83cc27569cf9bd27

then after sending it it actually gets the real unique ID.
Can be tested with the Test Webxdc app

this causes problems like ex. if you vote in a poll before sending it or join as "waiting for player..." in a chess game before sending the app in chat

[r10s]

the reason is probably that rfc724_mid is not set when get_webxdc_info() is called to query self_addr.

question is when to set rfc724_mid:

• on set_draft() it is too late
• maybe on get_webxdc_info(), we would need to write it back to the database then, however, which is a bit odd for a getter
• or when creating a new message object

another option might be to remove rfc724_mid from self_addr. iirc, that was considered also that time when we moved away from email-address = self-addr

[link2xt]

Creating rfc724_mid already when Message is created might also be good for #6426 because every message will be uniquely identified even when it is in the draft mode.