UID2-4391 Major refactor to improve performance and maintainability

[atarassov-ttd]

- Removed redundant separation of IO and Worker threads keeping a single pool IO worker threads
- Removed memory arena and custom memory management for IO buffers
- Removed half-baked support for multiple inflight output buffers
- Increased IO buffer size to 4096 bytes
- Simplified and cleaned up a lot of code
- Removed support for socks proxy in config as this was not an implemented feature
- Simplified socket implementation, especially around how sockets get closed in case of errors
- Make proxy a bit fairer by round-robining across all connections with a single IO operation at a time, instead of draining each socket in order
- Make support for handling async connections more explicit
- Add unit test coverage, most importantly - for the sockets IO logic
- Improved/corrected README

Manually verified through various tests as well as using tcptunnelchecker.

[scong-ttd]

• Increased IO buffer size to 1024 bytes

Shall we increase to more based on uid operator payload size? (or make it configurable)

[atarassov-ttd]

• Increased IO buffer size to 1024 bytes

Shall we increase to more based on uid operator payload size? (or make it configurable)

That was incorrect statement in the PR description, updated to 4096 which is the final number we settled on based on perf testing.