ConfdAsyncUDPServer: defer handling writes

Currently if we fail writing to the socket (perhaps because a signal was
delivered) we lose the data we were sending. Although this is not too
bad (it's udp, and data may get lost anyway) we try to avoid this by
just putting the packets to write in a queue and handling that through
the normal asyncore methods.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>