A RS died. The ServerShutdownHandler kicked in and started the logspliting. SpliLogManager
installed the tasks asynchronously, then started to wait for them to complete.
The task znodes were not created actually. The requests were just queued.
At this time, the zookeeper connection expired. HMaster tried to recover the expired ZK session.
During the recovery, a new zookeeper connection was created. However, this master became the
new master again. It tried to assign root and meta.
Because the dead RS got the old root region, the master needs to wait for the log splitting to complete.
This waiting holds the zookeeper event thread. So the async create split task is never retried since
there is only one event thread, which is waiting for the root region assigned.