protected boolean processFirstDataBlock() throws IOException {
      blockBuffer.rewind();
      readKeyValueLen();
      return true;
    }
Positions this scanner at the start of the file.
@return false if empty file; i.e. a call to next would return false and the current key and
        value are undefined.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderImpl.java