METS: Check existing children on ALTO node before linking children

Instead of trying to create links between alto nodes and their children, if they are present in the cache, we should only create missing links.

TODO, when processing ALTO nodes (mentioned in METS file):

• list children ALTO IDs
• get their Arkindex IDs from the cache
• if the node is not in the cache or no children are in the cache, publish the node and their children
• else, publish only missing children