Consider connection state changed events as valid and inform the client

ZooKeeper's Java client implementation considers connection state WatchedEvents
as valid. See src/java/main/org/apache/zookeeper/ClientCnxn.java#L416.

This is needed for upcoming changes like https://issues.apache.org/jira/browse/ZOOKEEPER-1607
in which an Observer will be able to change from CONNECTED to CONNECTED_RO without
dropping clients.

It shouldn't introduce any other side effects.