Reduce how often we have to poll the packet queue.

Reduce how often we have to poll the packet queue.

Packets are moved to the MeshLink thread via the packet queue. However,
each packet required a trigger byte to be sent to the event loop, requiring
more calls to select() than necessary. Now we make event loop signals level
triggered, and dequeue all enqueued packets at once.

This also adds debug log statements for the packet queue.