Async socket servers

[aleks-f]

• PollSet
  • review and fix any leftover problems
    (it's not an easy task to reconcile all platforms async socket I/O behavior)
  • HTTPServerConnection::onServerStopped does not halt poll #3985, PollSet not working as intended on Windows #3248
• UDPServer
  • Review (and modify, if necessary)
  • Add more extensive tests
  • Sample
• AsyncTCPServer
  • Basically, the same functionality as UDPServer for TCP (consider template)
  • Tests
  • Sample

Try to break the existing UDPServer as little as possible (ideally no breakage)

[matejk]

A library wepoll (https://github.com/piscisaureus/wepoll) implements epoll functions that wrap Win32 API.

[aleks-f]

https://github.com/pocoproject/poco/blob/devel/Net/src/wepoll.c

[matejk]

https://github.com/pocoproject/poco/blob/devel/Net/src/wepoll.c

Thanks, I noticed those files almost immediately after posting :)