Use priorities in Simulator

Currently the simulator ignores priorities. This means two things:

1. Bugs that are related to priorities (for example the death spiral that we saw
in production on FDB 3 and can reproduce on a test cluster in FDB 6) can't be
reproduced with the simulator. This makes testing and debugging much harder.
1. The behavior for certain workloads can be significantly different in the
simulator than it would be on a real cluster. This is a problem by itself.

We should find a way use priorities in the simulator to improve testing.