Session timeout detection can go wrong if the leader system time changes

                 Key: ZOOKEEPER-366
             Project: Zookeeper
          Issue Type: Bug
            Reporter: Benjamin Reed

the leader tracks session expirations by calculating when a session will 
timeout and then periodically checking to see what needs to be timed out based 
on the current time. this works great as long as the leaders clock progresses 
at a steady pace. the problem comes when there are big (session size) changes 
in clock, by ntp for example. if time gets adjusted forward, all the sessions 
could timeout immediately. if time goes backward sessions that should timeout 
may take a lot longer to actually expire.

this is really just a leader issue. the easiest way to deal with this is to 
have the leader relinquish leadership if it detects a big jump forward in time. 
when a new leader gets elected, it will recalculate timeouts of active sessions.

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