Only set parent references if changed when merging

Parent references on extracted content may not contain identifiers,
if they've been synthesised by an adapter without looking up from
the db first. Therefore by only setting if their URIs have changed,
the IDs will be maintained from the existing content.
Merge pull request #488 from atlasapi/kafka

Integrate Kafka
Allowing cors even for not preflight requests
Merge branch 'mbst-8719-improve-cors-filter' into stage