Optimize MapTask.MapOutputBuffer.spill() by not deserialize/serialize keys/values but use appendRaw

      DataInputBuffer keyIn = new DataInputBuffer();
      DataInputBuffer valIn = new DataInputBuffer();
      DataOutputBuffer valOut = new DataOutputBuffer();
      while (resultIter.next()) {
        keyIn.reset(resultIter.getKey().getData(), 
                    resultIter.getKey().getLength());
        key.readFields(keyIn);
        valOut.reset();
        (resultIter.getValue()).writeUncompressedBytes(valOut);
        valIn.reset(valOut.getData(), valOut.getLength());
        value.readFields(valIn);
        writer.append(key, value);
        reporter.progress();
      }