Rollback if previousLV >= LAYOUT_VERSION && prevCTime <= namenode.cTime.
   * Upgrade if this.LV > LAYOUT_VERSION || this.cTime < namenode.cTime Regular
   * startup if this.LV = LAYOUT_VERSION && this.cTime = namenode.cTime
Rollback if previousLV >= LAYOUT_VERSION && prevCTime <= namenode.cTime.
   * Upgrade if this.LV > LAYOUT_VERSION || this.cTime < namenode.cTime Regular
   * startup if this.LV = LAYOUT_VERSION && this.cTime = namenode.cTime | BlockPoolSliceStorage.java