Publicly expose task ID

[tcwalther]

I have my own implementation of your Queue code in my codebase which I wrote before I knew about this library. @groue told me about yours, and what I like about your code is that it has concepts like barriers which I don't have.

One thing that could be useful is to expose the task ID in a queue, with the ability to explicitly assign a task ID to a queue entry, and the ability to replace a queue entry by ID.

My use case is syncing code that syncs local changes to the backend. In my code, I reuse the ID of the entity I sync as queue entry ID, and when a user makes two changes in a row, I can cancel the first backend call and replace it with the newer one.

[mattmassicotte]

I'm so glad you brought this up!

Internally, the Queue type uses Task. So I think you can do a few of these things today.

let task = queue.addOperation {
}

print("identifier:", task.hashValue)

task.cancel()

Would it be possible to make use of this hashValue as a way of tying the work together?

Also, is the cancelation is an optimization? Task cancellation is inherently racy, but if you need to guarantee cancellation happens I'd have to think a lot harder about how to pull it off.