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* Fix in loadxl to PATCH on validate_only for items which already exist;
discovered during smaht-submitr testing.
* Fix in loadxl.normalize_deleted_properties which was creating/returning
a new (an_item) item, which was messing up determination of identifying
path for patch (as second_round_items comes from store but we had set uuid
in an_item which, without this fix, became a different object).
* Added skip_links feature to loadxl which will cause reference/link integrity
checking to be skipped altogether; this is (currently) only set by smaht-portal/
ingestion/loadxl_extensions.py for smaht-submitr, since that process already
does thorough reference integrity checking anyways (via structured_data).