Transaction conflict resolution

[novoj]

While we have designed entire transactional systems, this excludes the part where conflicts between multiple concurrent transactions are resolved and potentially conflicting transactions are rolled back. Because we need to test the transactional logic as soon as possible, we have moved the conflict resolution to a separate issue.

We want to have a tunable conflict resolution process. The developer should be able to

• set a default conflict resolution policy at the catalog level
• override that conflict policy at the entity collection level
• Use different conflict policies
  • deny changes in the same entity - all concurrent updates to the same entity will be denied
  • deny changes to the same
    • entity Attribute
    • entity related data
    • entity reference (by name)
    • entity prices
    • entity header

This conflict resolution should only apply to mutations that replace content. The `applyDelta' mutation should be excluded from this logic because it applies the change in a safe way that doesn't conflict with other updates.