Reduce number of calls to FlowNode#getEnclosingId

[lewisbirks]

A continuation of the investigation for #862, FlowNode#getEnclosingId was being called multiple times in NodeRelationshipFinder#addSeenNodes. Whilst the majority of the calculations were cached on the initial call, subsequent looks would still have to acquire locks and checks as it performs the non cached operations. Therefore reduce it to just once.

Testing done

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