hardcoded simple-priority queue as the only message queue

[pompon0]

There were multiple queue type options that you could select in the seid config.
It is known that the only simple-priority queue works reasonably and it should have been an implementation detail of the seid node anyway, so I've removed the other options removing the abstraction.
The additional benefit is that the unit tests are using the real thing instead of FIFO

Additionally:

• fixed tests which were broken by the change from FIFO to simple-priority
• refactored simple-queue so that send and recv always takes O(log n) (instead of O(n log n) when queue is full)
• removed the simple-queue's background task, because unit tests were not prepared to handle that (and it would be a shit ton of work to make them handle it)
• made simple-queue Send() explicitly non-blocking
• adjusted router to the changes
• limited the blast radius of "size * size" quirk by moving it from simple-queue to router (it would be hard to drop it entirely without semantic changes)
• added metric for dropped messages
• refactored listener code and connection management, so that connections don't get dropped for no reason
• replaced net.IP with netip.Addr, for easier handling (for example, netip.Addr can be used as map key)
• partially fixed race condition when 2 nodes try to connect to each other simultaneously (there are still conditions under which both will fail, but fixing that requires a more intrusive change)