Get the maintained data. In case of any ZK exceptions this will retry
establishing the connection (but not more than twice/minute).
getData is on the critical path, so make sure it is fast unless there is
a problem (network partion, ZK ensemble down, etc)
Make sure at most one (unlucky) thread retries and other threads don't pile up
while that threads tries to recreate the connection.
the last know version of the data
public void process(org.apache.zookeeper.WatchedEvent event)