@Override
    public void shipped() throws IOException {
      if (scanner instanceof Shipper) {
        SHIPPED_COUNT.incrementAndGet();
        ((Shipper) scanner).shipped();
      }
    }
Verifies that if a coproc returns an InternalScanner which implements Shipper, the shippped
method is appropriately called in Compactor.
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestCompactionWithShippingCoprocessor.java