Remove DevicePacketQueue, use a IO coroutine to send packets

When a Device is connected (ie: it has a link) we start a coroutine that reads from a channel where packets are enqueued. The channel has unlimited size, so enqueuing always succeeds. We cancel this coroutine when the last link disconnects.

This removes the ability of replacing packets that are still in the queue, but in my tests that code never triggered anyway.