BOOKKEEPER-668: Race between PerChannelBookieClient#channelDisconnected() and disconnect() calls can make clients hang while add/reading entries in case of multiple bookie failures (sijie & ivank via ivank)

git-svn-id: https://svn.apache.org/repos/asf/zookeeper/bookkeeper/trunk@1515769 13f79535-47bb-0310-9956-ffa450edef68