Fix resources & priorities when tasks are submitted in several submits with delay.

[spirali]

This is the next attempt to fix #848 .

[Kobzol]

The tests seem like they could be a bit flaky, it's better to use wait_until rather than sleeps.

[spirali]

I have added some additional tests and they triggered the same assert as in #853. It is fixed now.

For the record: The bug is actually relatively old, the first fix of #848 did not introduce it, just expose it.