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

rdiff_backup/pool.py

@@ -57,6 +57,7 @@ class Pool(object):
 			     max_taskqueue_size=0, max_jobqueue_size=0):
 		if processes is None:
 			processes = cpu_count()
+		processes = 1 # XXX workaround until race conditions are fixed.
 		self.processes = processes
 
 		# Init queues