tunnel: improve vpn performance

[pymq]

See the individual commits for more details. This PR improves bandwidth for high-latency networks by 1.5-2x and reduces overall tunneling overhead, which should also improve peak bandwidth for low-latency networks as well (though this was not measured in the benchmarks).

Benchmark results for the simulated QUIC network with simnet. Manual testing produced similar results.

Before:

+-----------------------------+---------+-----------------+-------------------+-------------+-------------+
|          SCENARIO           | LATENCY | BANDWIDTH LIMIT | ACTUAL THROUGHPUT | UTILIZATION | PACKET LOSS |
+-----------------------------+---------+-----------------+-------------------+-------------+-------------+
| Fiber_200Mbps_1ms           | 1ms     | 200 Mbps        | 149.67 Mbps       | 74.83 %     | 99.55 %     |
| LongDistFiber_200Mbps_200ms | 200ms   | 200 Mbps        | 35.29 Mbps        | 17.65 %     | 99.91 %     |
| Fiber_100Mbps_1ms           | 1ms     | 100 Mbps        | 91.59 Mbps        | 91.59 %     | 99.78 %     |
| LongDistFiber_100Mbps_200ms | 200ms   | 100 Mbps        | 29.17 Mbps        | 29.17 %     | 99.94 %     |
| Cable_10Mbps_1ms            | 1ms     | 10 Mbps         | 9.73 Mbps         | 97.26 %     | 99.98 %     |
| Cable_10Mbps_10ms           | 10ms    | 10 Mbps         | 9.74 Mbps         | 97.36 %     | 99.98 %     |
| Cable_10Mbps_100ms          | 100ms   | 10 Mbps         | 8.34 Mbps         | 83.44 %     | 99.98 %     |
| Cable_10Mbps_200ms          | 200ms   | 10 Mbps         | 7.47 Mbps         | 74.68 %     | 99.98 %     |
| LongDistCable_10Mbps_300ms  | 300ms   | 10 Mbps         | 5.95 Mbps         | 59.51 %     | 99.99 %     |
| Cable_50Mbps_1ms            | 1ms     | 50 Mbps         | 48.23 Mbps        | 96.46 %     | 99.88 %     |
| Cable_50Mbps_10ms           | 10ms    | 50 Mbps         | 47.96 Mbps        | 95.93 %     | 99.89 %     |
| Cable_50Mbps_100ms          | 100ms   | 50 Mbps         | 36.21 Mbps        | 72.42 %     | 99.91 %     |
| Cable_50Mbps_200ms          | 200ms   | 50 Mbps         | 25.02 Mbps        | 50.04 %     | 99.94 %     |
| LongDistCable_50Mbps_300ms  | 300ms   | 50 Mbps         | 18.32 Mbps        | 36.64 %     | 99.95 %     |
| DSL_20Mbps_25ms             | 25ms    | 20 Mbps         | 18.18 Mbps        | 90.90 %     | 99.95 %     |
| LTE_30Mbps_40ms             | 40ms    | 30 Mbps         | 26.42 Mbps        | 88.06 %     | 99.93 %     |
+-----------------------------+---------+-----------------+-------------------+-------------+-------------+

After:

+-----------------------------+---------+-----------------+-------------------+-------------+-------------+
|          SCENARIO           | LATENCY | BANDWIDTH LIMIT | ACTUAL THROUGHPUT | UTILIZATION | PACKET LOSS |
+-----------------------------+---------+-----------------+-------------------+-------------+-------------+
| Fiber_200Mbps_1ms           | 1ms     | 200 Mbps        | 152.58 Mbps       | 76.29 %     | 99.87 %     |
| LongDistFiber_200Mbps_200ms | 200ms   | 200 Mbps        | 84.32 Mbps        | 42.16 %     | 99.93 %     |
| Fiber_100Mbps_1ms           | 1ms     | 100 Mbps        | 90.64 Mbps        | 90.64 %     | 99.93 %     |
| LongDistFiber_100Mbps_200ms | 200ms   | 100 Mbps        | 58.03 Mbps        | 58.03 %     | 99.95 %     |
| Cable_10Mbps_1ms            | 1ms     | 10 Mbps         | 9.74 Mbps         | 97.45 %     | 99.99 %     |
| Cable_10Mbps_10ms           | 10ms    | 10 Mbps         | 9.75 Mbps         | 97.50 %     | 99.99 %     |
| Cable_10Mbps_100ms          | 100ms   | 10 Mbps         | 8.58 Mbps         | 85.76 %     | 99.99 %     |
| Cable_10Mbps_200ms          | 200ms   | 10 Mbps         | 7.94 Mbps         | 79.42 %     | 99.99 %     |
| LongDistCable_10Mbps_300ms  | 300ms   | 10 Mbps         | 7.08 Mbps         | 70.76 %     | 99.99 %     |
| Cable_50Mbps_1ms            | 1ms     | 50 Mbps         | 47.71 Mbps        | 95.41 %     | 99.96 %     |
| Cable_50Mbps_10ms           | 10ms    | 50 Mbps         | 47.60 Mbps        | 95.20 %     | 99.96 %     |
| Cable_50Mbps_100ms          | 100ms   | 50 Mbps         | 37.31 Mbps        | 74.61 %     | 99.97 %     |
| Cable_50Mbps_200ms          | 200ms   | 50 Mbps         | 30.52 Mbps        | 61.05 %     | 99.98 %     |
| LongDistCable_50Mbps_300ms  | 300ms   | 50 Mbps         | 25.11 Mbps        | 50.23 %     | 99.98 %     |
| DSL_20Mbps_25ms             | 25ms    | 20 Mbps         | 18.29 Mbps        | 91.43 %     | 99.99 %     |
| LTE_30Mbps_40ms             | 40ms    | 30 Mbps         | 26.38 Mbps        | 87.92 %     | 99.98 %     |
+-----------------------------+---------+-----------------+-------------------+-------------+-------------+