DPL-nnn Acts_as_dag is unmaintained

[JamesGlover]

Describe the Housekeeping Acts_as_dag is used to maintain the asset_links relationships, but it is unmaintained and hasn't received an update since 2014.

It still works, but should be considered a liability.

We've currently got a bit of a mess of ways of representing relationship graphs, each optimised for a different scenario. I've been wanting to migrate all of this to Neo4J for a while (tracking relationships at the aliquot level) but this is a pretty large undertaking.

Blocking issues Describe any other issues or tickets that may be blocking this change.

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