background fetch worsening

[r10s]

while playing around with #1428 , i am getting pretty stable wakeups every 10 minutes, however, no messages are fetched in these slots (testing on iphone7 with
ios 15.2.1).

i am pretty sure that worked before, but i cannot say when it broke, eg. if 1.26 is fine or not..

we got similar reports also far before, eg. from @dignifiedquire , but we could not get a grip on that, so that issue may be even older.

cmp. #1157

[cyBerta]

We should also double check it's not related to db encryption

[r10s]

after little more observing, it seems sometimes messages are fetched in background, but not as often as possible it seems.

as by the new notification-info in the connectivity-view, we know the wakeup is called quite regulary - might be, that it does not finish within the 10 seconds before we leave background (there is an issue to run as long as needed #1157 , however, even without, there seems to be something too slow)

[cyBerta]

this needs to be retested with latest core now, that deltachat/deltachat-core-rust#3041 and deltachat/deltachat-core-rust#3040 has been merged

[r10s]

seems, fetching is again available as before, at least on my device.

reconnection still takes longer, roughly double time, i did not measure in details, but that is in sync with measurements at deltachat/deltachat-core-rust#3035 (comment)).

closing this issue as there are more concrete ones for the remaining options, deltachat/deltachat-core-rust#3035 and #1157