Kafka server can miss zookeeper watches during long zkclient callbacks

On getting a zookeeper watch, zkclient invokes the blocking user callback and only re-registers the watch after the callback returns. This leaves a possibly large window of time when Kafka has not registered for watches on the desired zookeeper paths and hence can miss important state changes (on the controller). In any case, it is worth noting that even though zookeeper has a read-and-set-watch API, there can always be a window of time between the watch being fired, the callback and the read-and-set-watch API call. Due to the zkclient wrapper, it is difficult to handle this properly in the Kafka code unless we directly use the zookeeper client. One way of getting around this issue is to use timestamps on the paths and when a watch fires, check if the timestamp in zk is different from the one in the callback handler.