handle webxdc updates for not downloaded instances (#3487)

* 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

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 ### Fixes
 - replace musl libc name resolution errors with a better message #3485
+- handle updates for not yet downloaded webxdc instances #3487
 
 
 ## 1.88.0