refactor(cell): change PacketQueue::dequeue to dequeue_at(ts)

[CepheusC]

Summary

Add an Option<Instant> timestamp parameter to PacketQueue::dequeue() to support time-aware queue operations. The CoDel queue uses this parameter to avoid Instant::now() calls, while simple queue implementations (DropHead, DropTail,
Infinite) ignore it with None.

Usage

This change lays the groundwork for PR #197 (redpie branch) by providing the necessary timestamp plumbing in the PacketQueue trait. The subsequent PR will make use of this parameter to implement its feature.

[BobAnkh]

This shall never be an optional argument. And please rename the interface from dequeue to dequeue_at. @CepheusC

For simple queues, they can just ignore the value (but we shall still pass the actual value from the cell), as they do not require it. But the semantics of this interface should be fixed to: the cell wants to dequeue a packet at the timestamp, as in our design philosophy, the queue is a passive-driven component.

[BobAnkh]

LGTM

[BobAnkh]

Wait for extra two approvals (from @Centaurus99 , @un-lock-able or @Lethe10137 ) to merge this.

[un-lock-able]

LGTM

[Centaurus99]

In addition, please squash the changes into a single commit and update the PR title accordingly. @CepheusC

[BobAnkh]

In addition, please squash the changes into a single commit and update the PR title accordingly. @CepheusC

We could squash ourselves when merging, so he doesn't need to worry about that and you can have a clear history to review changes. Also, we should try to compact the head into 50 characters instead of spliting into two lines.

[Centaurus99]

LGTM.