0.10.1, 0.11.2, 0.12.2
Name-node can have multiple directories to store the name space image and edits.
During startup the name-node selects the latest image and reads it in memory.
fstime stores the time of the latest checkpoint.
If one of the directories becomes inaccessible during normal operation the name-node
excludes it from the list, and never writes anything into it until restarted.
Now if the the name-node restarts after excluding one of the directories, and if that
excluded directory contained the latest image, then all modifications of the name space
recorded in edits files in other directories starting from the moment the directory was
excluded will be lost.
We should update fstime in all remaining good directories after the exclusion.