Add new FileSystem API to support asynchronous method calls

Add a new API, namely FutureFileSystem (or AsynchronousFileSystem, if it is a better name). All the APIs in FutureFileSystem are the same as FileSystem except that the return type is wrapped by Future, e.g.

  //FileSystem
  public boolean rename(Path src, Path dst) throws IOException;

  //FutureFileSystem
  public Future<Boolean> rename(Path src, Path dst) throws IOException;

Note that FutureFileSystem does not extend FileSystem.