declare background task later and redeclare them as needed

[r10s]

currently, we are declaring the background task already on startup, in installEventHandler() and getting a warning as follows:

2021-04-06 18:22:37.504670+0200 deltachat-ios[5801:4433962] [BackgroundTask] Background Task 1 ("Called by deltachat-ios, from $s13deltachat_ios11AppDelegateC19installEventHandleryyFyycfU_"), was created over 30 seconds ago. In applications running in the background, this creates a risk of termination. Remember to call UIApplication.endBackgroundTask(_:) for your task in a timely manner to avoid this.

so, ios, seems to use this also as a time-based-marker, might be that this was different in older os.

https://developer.apple.com/documentation/uikit/uiapplication/1623031-beginbackgroundtask say, the background task can be declared at "any point", however, also speaks of "mark the beginning". i suggest to move the registering to start of applicationDidEnterBackground.

moreover, the current logic allows only one background task at all - then the app has to be terminated and restarted to have another background task. in practise, however, apps are suspended nowadays.