[WIP] close database on suspend

[r10s]

maybe an alternative to #1206, targets #1202; let's discuss that tomorrow in an a/v :)

[r10s]

well, it is not as easy as this pr suggests:

• i got a crash releated to that pr, for whatever reason, did not dived into that further, maybe related to
• creating/deleting context objects, we would also have to shutdown event handlng and restart it, that has issues on its own (eg. Event loop stopping is not thread-safe deltachat-core-rust#2280)

so, maybe do not go that way now, that would only interchange issues :)

[r10s]

replaced by #1208