Seems like this user land tcp stack could be the basis of types of c10m servers. Are tbere existenting userland tcp stacks available? I'm not familiar with any.
There's also some work showing that you can achieve very high performance with kernel TCP: https://www.usenix.org/conference/osdi14/technical-sessions/...
A good general purpose stack is the 6windgate stack. I know nothing about it personally, but I know that a lot of people do use it successfully.