Implement fully async I/O through libev, along with some fixes

[colfrog]

• Message processing is now queue-based
• Add a timeout watcher to the event loop
• Push message processing to handle_message, outside of the event loop
• Changed check_socket for verify_socket
• Fix some warnings
• Push the call to setnonblock to after the TLS setup to avoid an error
• Check whether the socket is invalid only after we're done looping over addrinfo
• Check for errno after irc_server_connect