Performance fixes for crunch's join

TupleWritable.scala

  /**
   * {@inheritDoc}
   */
  @SuppressWarnings("unchecked")
  // No static typeinfo on Tuples
  public void readFields(DataInput in) throws IOException {
    int card = WritableUtils.readVInt(in);
    if (values == null || values.length != card) {
      values = new Writable[card];
    }
    written = WritableUtils.readVLong(in);
    Class<? extends Writable>[] cls = new Class[card];
    try {
      for (int i = 0; i < card; ++i) {
        if (has(i)) {
          cls[i] = lookupClass(Text.readString(in));
        }
      }
      for (int i = 0; i < card; ++i) {
        if (has(i)) {
          if (!cls[i].isInstance(values[i])) {
        	values[i] = cls[i].newInstance();
          }
          values[i].readFields(in);
        } else {
          values[i] = null;
        }
      }
    } catch (ClassNotFoundException e) {
      throw (IOException) new IOException("Failed tuple init").initCause(e);
    } catch (IllegalAccessException e) {
      throw (IOException) new IOException("Failed tuple init").initCause(e);
    } catch (InstantiationException e) {
      throw (IOException) new IOException("Failed tuple init").initCause(e);
    }
  }

  private Map<String, Class<? extends Writable>> writableClassCache = new HashMap<String, Class<? extends Writable>>();

  private Class<? extends Writable> lookupClass(String readString) throws ClassNotFoundException {
	  if (!writableClassCache.containsKey(readString)) {
		  writableClassCache.put(readString, 
				  Class.forName(readString).asSubclass(Writable.class));
	  }
	  return writableClassCache.get(readString);
  }