Merge "Iterate through the sorted file map when finding transfers"

    max_blocks_per_transfer = int(cache_size * split_threshold /

                                  self.tgt.blocksize)

    empty = RangeSet()

    for tgt_fn, tgt_ranges in self.tgt.file_map.items():

    for tgt_fn, tgt_ranges in sorted(self.tgt.file_map.items()):

      if tgt_fn == "__ZERO":

        # the special "__ZERO" domain is all the blocks not contained

        # in any file and that are filled with zeros.  We have a