public void initialize(InputSplit inputsplit, TaskAttemptContext context)
    throws IOException, InterruptedException {
    if (context != null) {
      this.context = context;
    }
    restart(scan.getStartRow());
  }
Build the scanner. Not done in constructor to allow for extension.
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/TableRecordReaderImpl.java