add UDP support, and TCP listen support ipv6 now
If recv one udp message, try read next immediately rather than until next epoll_wait
remove unused code
bugfix: use ai_list instead of ai_hints
Merge branch 'udp' into dev
Merge branch 'dev'
add comment
Merge pull request #204 from cloudwu/dev

Dev
Merge remote-tracking branch 'cloudwu/master'