The value of the state path for a partition in ZK can be updated by both
the controller (for choosing the new leader and potentially shrinking Isr)
and the leader replica (for expanding/shrinking Isr). After the controller
modified the state path, we don't want the old leader to modify Isr
anymore.
I'm not quite sure how we get into this state, but we've seen this a few times
now. Basically, one of our brokers (broker 1 in this case) gets into a state
where ISR updates fail forever:
[2013-10-16 06:19:12,448] ERROR Conditional update of path
/brokers/topics/search-gateway-wal/partitions/5/
