non-blocking system DNS resolver

 Net::DNS::Native provides several methods for host name resolution.
 It is designed to be used with event loops. All resolving are done by
 getaddrinfo(3) implemented in your system library. Since getaddrinfo()
 is blocking function and this class doesn't want to block, calls to
 this function will be done in separate thread. This class uses system
 native threads and not perl threads. So overhead shouldn't be too big.