@Override
  public FSDataOutputStream append(Path f, int bufferSize, Progressable progress)
    throws IOException {
    return real.append(f, bufferSize, progress);
  }
Create a non-abstract "proxy" for FileSystem because FileSystem is an abstract class and not an
interface. Only interfaces can be used with the Java Proxy class to override functionality via an
InvocationHandler.
hbase-server/src/test/java/org/apache/hadoop/hbase/wal/FileSystemProxy.java