* save webxdc-updates for not yet downloaded messages, that are probably webxdc instances then
* test webxdc updates received while instance is not yet downloaded
* keep msg_id on downloading messages
keeping msg_id on downloading messages
has the advantage that webxdc updates and other references to the msg_id
can be processed as usual.
if a message expands to multiple msg_id,
the last one is kept,
however, this does not affect webxdc at all.
(alternatives may be to update `msgs_status_updates`
but that seems more complicated and even less elegant,
another alternative would be to use different keys (eg. `rfc274_mid`),
but that also seems not to be much easier and would waste space as well.
also both alternatives would need adaption for other foreign keys)
* update CHANGELOG
* do not emit WebxdcStatusUpdate event in case the message is not yet downloaded
* move DELETE/UPDATE to an transaction
* make merge_msg_id() a little less confusing
* use some webxdc-update-param from placeholder
(the placeholder may be updated,
the just downloaded messages is not)
* more precise function name
* test not directly downloading status updates
* test not directly downloading mdn
