MTU Path discovery

[Wunka]

Closes #819

This has still one big problem:

• Only client or server increase their mtu estimate. For example if client increases it, the server can't anymore for whatever reason. It seems like the probe is send, the other gets it and does a confirmation, but that confirmation is never received.
• After fixing the above issue, debug information needs to be deleted and the setting for mtu also needs to be removed. Also the timers and in general constants need to be adjusted to right values. Currently they are made better for debugging.

Changes:

• Connections now have a probingState. This + the connectionState about represent the State machine in RFC 8899. The only state missing is the Error state.
• The slow channel now also acts as a ProbeChannel. That means it will query on the connections probingState if it should send a probe and if yes it queries which size it should have and gives the probingState the SequenzIndex of the probe. With this we can then identify when a confirmation is for the probe
• Probes are for now only send when no other should be send over the slow channel, as this is easier to implement
• Probes are done with the mtu estimate + 50. (here there is no number mentioned in the RFC)
• On a double loss of packets in general, the mtu estimate is reseted again to the mtuMin. Here we can maybe in the future make better heuristics. But I don't think networking is such a bottleneck that we need that.
• variables where in general created with the corresponding RFC 8899 name in mind.

[Wunka]

So, a problem we have:
We kinda want to pad the normal messages, but for that we would have to go very weird around our Sendbuffer. As we don't need probes to be always received, so resending / confirmation is something we don't that much care about. That's why in this pr I have gone the way of doing a seperate channel for probe messages, which don't have a send / receive buffer. The other side just sends a confirmation packet that it was received and nothing more. This sadly means we can't pad messages, but instead have to send special probe messages which only contain nonsense data.

[IntegratedQuantum]

This sadly means we can't pad messages, but instead have to send special probe messages which only contain nonsense data.

At the end of the data it probably doesn't matter. Both techniques have performance impacts, and as long as the algorithm guarantees that only a small fraction of the throughput is probes, it shouldn't matter.