Sets cause non-determinism in the WorkerState

The recent worker state machine refactor made it possible to achieve truly deterministic and repeatable behaviour in worker unit tests (at least for as long as integration with the scheduler is not needed).

There is a big element of randomness left however, which was highlighted by #6587: anything that goes into a set and then is extracted from it is in random order.

For example, it is now possible to put many tasks in the fetch queue and get the exact same Instructions as a response every time - *unless* any of those tasks counts more than one worker in the `TaskState.who_has` set. In that case, every time you restart the interpreter, `WorkerState.handle_stimulus` will give you a different result.

If you transition a task to memory and that task has 2 or more dependents, they will transition from waiting to ready in random order (which in turn will cause `WorkerState.log` to be in random order), but their place in `WorkerState.ready` will be deterministic, thanks to the fact that `generation` is appended to task priority, so the `Execute` instructions returned by `handle_stimulus` will be deterministic.
However, if the dependents have resource constraints, they will be executed in random order, because `WorkerState.constrained` is a simple deque that ignores priority (which is a separate problem, out of scope for this ticket).

There are likely other use cases.

# Proposed design 1
- Replace all sets in the WorkerState with a custom insertion-sorted set (likely a wrapper around `dict.keys()`).
- Care needs to be taken for performance of set arithmetics, which are functionally implemented by just subclassing `collections.abc.MutableSet` but are frequently implemented inefficiently.
- Care needs to be taken in msgpack serialization and in `recursive_to_dict` to treat these objects properly.
- Conveniently, this would also allow to better encapsulate `scheduler.WorkerState.has_what`, which is an insertion-sorted set (this feature is needed by `replicate`).

# Proposed design 2
- Add a unique generation to the priority of all dependencies to be fetched by `compute-tasks` and `acquire-replicas`.
- Add a test to `validate_state` that checks that no two tasks have the same priority.
- Replace the `WorkerState.constrained` deque with a heap.
- Carefully review all other cases of sets for potential non-determinism.

CC @fjetter @gjoseph92 @graingert