Enforce _has_welcome being False when connecting to a server

A few days ago, kaoz has been sleeping on a server after a reconnection failed:

    Feb 22 15:57:58 kaoz[2231]: [INFO] kaoz.publishbot: disconnect event received
    Feb 22 15:58:15 kaoz[2231]: [INFO] kaoz.publishbot: connecting to chat.freenode.org:6667...
    Feb 22 15:59:21 kaoz[2231]: [ERROR] kaoz.publishbot: Error connecting to chat.freenode.org: Couldn't connect to socket: [Errno 110] Connection timed out

To avoid such a situation, make it impossible for is_connected() to ever
return True so that _check_connect is still run after reconnection failure.

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>