Switching to c-ares

Well, writing my own async DNS resolver proved to be more difficult than expected. However, I have just converted the IRCd to use the c-ares resolver library. This library seems to be a lot faster than UDNS, and I have so far not experienced any unexplained errors.

One issue that does need to be addressed is DNS queries finishing after a client has exited (and thus their pointer now invalid) and attempting to use clients data. To resolve this I will have the DNS callback functions report finished requests in a "DNS Query queue", which will later be checked for pending clients requests.