util: add DelayQueue::try_remove

[erwanor]

Adds a safe method to remove an item from the queue.

Fixes: #3558

Motivation

DelayQueue does not expose any way to check that an item is present in its slab. On the other hand, remove will panic if a client attempts to remove a key that is not in the queue. As a result, client libraries that have to avoid panics must maintain an extra HashSet around the queue.

Solution

Add a try_remove method which performs an internal membership check before calling remove. If a client attempts to remove a key that isn't there, try_remove will return None instead of panicking.

Since try_remove is the only method of its kind, I have elected to make it return an Option<Expired<T>> instead Result<Expired<T>, SomeNewErrorEnum>, at least for now.

[Darksonn]

This looks reasonable.

[Darksonn]

Thanks!