netmodel: some network programming models

referenced from chapter 6 in chenshuo's book

all codes are from chenshuo's book and UNPv3 chinese version

NOTE: all codes are no error handlings, just for teching or for my personal recording purpose, thanks:)

0. accept+read/write

1. accept+fork

2. accept+thread

3. prefork

4. pre threaded

5. poll(reactor)

6. reactor + thread-per-task

7. reactor + worker thread

8. reactor + thread pool

9. reactors in threads

10. reactors in processes

11. reactors + thread pool(combination of model 8 and model 9)