Optimise: simplify file structure used for file_relation_data

[chrisbc]

The current design has these relations saved independenty to unique object.json files. So, for every link we have:

  ObjectA.son  <-> Relationship.json <-> ObjectB.json
   - other props       - Role           - other props

this is a very flexible solution, supporting many-to-many relationships and also relationship properties (e.g ROLE) . The price is that the extra file read/writes make certain API operations overly slow and IO intensive.

Proposed:

 ObjectA.json                        <-> ObjectB.json
  - relatedTo [(objectID, Role)]         - relatedTo [(objectID, Role)] 
  - other props                        - other props

Snags:

• as currently the Relationship object is a graphql Node, so what do we have that relies on the abilitiy to resolve Node id? If nothing then there's no client impact
• old data will need to be migrated
• now to test the stability and actual performance impacts of this change

[chrisbc]

suggested approach

• FileRelation - remove id (it's no longer a Node) , find/fix impacted tests
• remove node storage ops from data.FileRelationData class, migrate props into the related objects

fix tests, code review.
later .... see #91