thrgd

This is seriously such a nice fix for a super annoying problem @RichieViscardi mentioned, which began appearing recently. I tried pushing this to the firebase-support, but they don't really understand...

@RichieViscardi How have you been doing this? Unfortunately this does not always prevent from multiple deliveries on my side.. reduces this issue drastically though.

This issue is still not fixed for batch-requests. The `sendAll` function still uses 10s.