IRCListener ServerName configuration is read multiple times

[strictlymike]

The IRCListener class uses .get('servername', 'localhost') to get its ServerName configuration value the first time in order to make the setting optional with the default value 'localhost'; but its ThreadedTCPRequestHandler accesses self.server.config['servername'] directly, resulting in a KeyError that winds up closing connections to the IRC service. Replacing direct references to self.server.config['servername'] with self.server.servername in the ThreadedTCPRequestHandler seems to resolve this and make the ServerName setting truly optional.