There is simply no way to guess data serials and instead of producing random
values, the only solution is to retrieve the values from storages.

There are still differences for data serials between FileStorage and NEO:
- NEO always resolves to original serial, to avoid any indirection
  (which slightly speeds up undo at the expense of a more complex pack code)
- NEO does not make any difference between object deletion and creation undone
  (data serial always null in storage)
It has to be decided whether NEO implementation should be changed about this.

Apart from that, conversion database back from NEO should be fixed.
testExportFileStorageBug passes and there was in fact no FileStorage bug.

Another change is that iterator does not trash the client cache anymore.