Push notifications were sporadically not sent on iOS

[matthesatox]

Describe the bug
Push notifications were sporadically not sent

To Reproduce
Occurrence: 5 out of 10 times]
Steps to reproduce the behavior:

1. User#A: Put the app to background
2. User#B: Send message to User#A

Expected behavior
A push notification should appear on mobile of user#A

Screenshots
If applicable, add screenshots to help explain your problem.

App information (please complete the following information):

• App version: 0.600.0 (513)
• Plugin version:
• Delta Chat Core version:
• Flutter version:

Smartphone (please complete the following information):

• Device: iPhone 11 Pro Max
• OS: iOS 13.4

Additional context
Add any other context about the problem here.

[phranck]

This is a known issue and currently not fully implemented/hardened. Planned for Beta-7 (see internal ticket).