public boolean hasBeenModified(long mtime) {
      return this.lastModified < mtime;
    }
Check if the snapshot directory has been modified
@param mtime current modification time of the directory
@return <tt>true</tt> if it the modification time of the directory is newer time when we
        created <tt>this</tt>
hbase-server/src/main/java/org/apache/hadoop/hbase/master/snapshot/SnapshotFileCache.java