Correctly replace a channel if it is closed but the channelDisconnected event has not fired on it.
 - Correctly replace a channel if it is closed but the channelDisconnected event has not fired on it.
 Changed the code to check if the channel is closed before returning it. If the channel is closed, we replace it during the getXChannel() operation. More details on the investigation can be found at https://jira.twitter.biz/browse/PUBSUB-116#comment-789617

RB_ID=134670