This is a follow up of
HBASE-9161. Currently, if a region is in transition, we ignore any unexpected zk assign event and log a warning. For example, a zk closing event will be ignored if the region is pending open actually. However, if the region is not in transition, we will accept such events. For events not related to split/merge, we should ignore them and log a warning too, to tighten up the region state transition. During normal operation, this (ensuring region is in transition before accepting transition related events) is fine. During master failover, this is fine as long as we don't do region transition bypass ZK. We should also make sure to watch assignment znodes after user region states are recovered, so that we don't log many such warnings.