Add lossy mode

[hizani]

This PR adds stateless lossy mode which is enabled by -l option.
Lossy mode server is compatible with lossless mode clients and vice versa.

I can't actually test it because -L option doesn't work in master build at the time the PR was created, but ping mode works fine.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[zoff99]

@hizani
the first byte of data for tox custom packets is the packet ID. which is globally unique.
please register all IDs here:

https://github.com/zoff99/toxcore_custom_packets_registry

thank you.

[hizani]

@zoff99
Done. I've added only the lossless ID, because I don't know if this PR will ever be merged.

[gjedeer]

@hizani Can you tell me the motivation for it? I thought about adding lossy packets in the far future, but for forwarding UDP.

TCP retransmission won't work at this level, so you'd just have incomplete TCP transmission if I understand correctly. Don't get me wrong, I'm not against merging this but please write why someone may need this.

[hizani]

@gjedeer The motivation is to reduce latency caused by waiting on SENDQ here. It can help latency-sensitive cases like device monitoring, multiplayer gaming etc.

As far as I understand it, lossy packets can work over TCP without problems but there won't be any ordering on the toxcore level.

[gjedeer]

Let me know when you test it on a lossy network and get some of these messages in the log. I'm pretty sure any software working over TCP will not work correctly when it loses random part of the stream. One exception I can think of is maybe some video player like mpv or vlc, these can handle completely malformed input.