Tasks with resource constraints ignore priority

[crusaderky]

Worker._ensure_computing is implemented as follows:

while self.constrained and there are free threads and there are free resources:
    task = self.constrained.popleft()
    compute(task)
while self.ready and there are free threads:
    _, task = heapq.heappop(self.ready):
    compute(task)

This means that, once tasks with resource constraints land on a worker, they are executed in a strictly FIFO order.
What's more, they are always prepended to tasks without constraints (as long as the worker resource isn't depleted).
Effectively, this is like setting priority=999999 on the client.

Note: I did not analyse the algorithm on the scheduler side. A review is recommended.

Related issues:

• Incompatibility between allow_other_workers and resources #4792
• Workers with multiple resources may be underutilized #6136