Make queue size configurable & drop oldest packets first

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This PR contains three commits:

1. Make the queue size configurable. In my program, the server sometime starts on-demand, and the client buffers a significant number of packets by default, which causes a bit of flooding
2. Replaces a conditional with an assertion. I'd happily revert if this desired by maintainers. This code would probably be a bit more readable if dequeue_packet and requeue_packet were inlined into their callsite, which I'd also happily contribute
3. Drop oldest packets first. I believe this is reasonable (think TCP retransmits), but I'd be happy to keep the current behaviour if wanted

Side-note: Is there a way to run only tests that don't required sudo?