Speed up akill xline checks

Cache xline nick, user, host, etc instead of rebuilding it everytime its
requested. Store users ip in sockaddr form and not string form to
prevent having to rebuild sockaddrs when checking xlines.

Also do not try to convert empty config values in Config::Get as this
can be rather common if a non string configuration value is not set, and
the cost of the ConvertException is great.

include/modules/os_session.h

@@ -7,7 +7,7 @@ struct Session
 	unsigned count;                 /* Number of clients with this host */
 	unsigned hits;                  /* Number of subsequent kills for a host */
 
-	Session(const Anope::string &ip, int len) : addr(ip, len), count(1), hits(0) { }
+	Session(const sockaddrs &ip, int len) : addr(ip, len), count(1), hits(0) { }
 };
 
 struct Exception : Serializable