Add callback LWS_CALLBACK_SERVER_NEW_CLIENT_INSTANTIATED 

This new callback allows the user to read or change the socket properties just after it was instantiated by the library. For example, to add a timer to drop clients who never request anything (but keep the TCP session open), so they can not hold a file descriptor for too long, make poll queue longer, etc. as I was trying to do (by the wrong way) in issue #48.

I think it can be very for another people so, please, merge it.

Best regards,