pool: disable parallelism

This seem to have race conditions and not ready for production.

We observed some IndexError caused in

Traceback (most recent call last):
  File "python2.7/lib/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "python2.7/lib/python2.7/threading.py", line 754, in run
    self.__target(*self.__args, **self.__kwargs)
  File "rdiff_backup/pool.py", line 47, in worker
    value = task.func(*task.args)
  File "rdiff_backup/restore.py", line 65, in get_diff
    diff = MirrorStruct.get_diff(mir_rorp, target_rorp)
  File "rdiff_backup/restore.py", line 292, in get_diff
    file_fp = cls.rf_cache.get_fp(expanded_index, mir_rorp)
  File "rdiff_backup/restore.py", line 392, in get_fp
    rf = longname.update_rf(self.get_rf(index, mir_rorp), mir_rorp,
  File "rdiff_backup/restore.py", line 388, in get_rf
    else: del self.rf_list[0]
IndexError: list assignment index out of range

apparently because the underlying code is not thread safe in the first place.