[WIP] Experiments with worker threads

[FelixMcFelix]

We have some aspects of packet delivery that make worker threads and/or full-fledged squeues, interesting to experiment with:

• Packets for a logical port will arrive from both the underlay and its VNIC -- likely two threads with different CPU bindings.
• Ports are currently locked via mutex, such that we want to serialise packets bound for a port to be processed by the same kthread.
• Packets arriving on the VNIC backing an OPTE port are delivered one packet at a time. (I have not yet dug into this behaviour on the illumos side).
• Packets arriving on the Underlay will be delivered one-at-a-time until we can finalise/implement the ongoing illumos flows design work.

Currently this branch includes some Rust wrappers around kcondvars and thread creation/binding in a kernel module context (and a basic MPSC with worse contention than I'd like). I'm using this as a space to hash out some ideas in this space, even if the lifetime of this work is inevitably temporary.