This speeds up 2 operations that were horribly slow (linear scan
of source database, from the beginning or from the end):
- read access to not yet migrated data
- resumption (restart of storage node whereas import was not finshed)

Testing will tell if this patch is enough, or if more efficient solutions are
required, like:
- building a preliminary {tid->file_pos} index while the source DB is still
  in use (in this case, NEO may have to deal with a small gap at the end)
- disabling ZODB features requiring data_serial (undo)