finer control over future_lapply()

I'd like to implement the following using futures, but it doesn't seem possible yet?

- [ ] Apply a function over a list of objects and get a list of futures. The tasks will start running immediately up to the number of workers and, if there are more tasks than workers, the remainder are queued for running using load-balancing.
- [ ] Be able to iterate over the list of futures and check if the future is resolved, running, or queued.
- [ ] Be able to cancel futures that are running or queued. When a future is cancelled, the next one in the queue starts executing.