Fix memento/rebind of AbstractController

- serverPoolTargets has entities as keys, so needed to fix
  MementoTransformer.transformIdsToEntities so it converted
  keys and values correctly.
- Can revisit this in the future for instead using serialised 
  form of an entity-ref, rather than just String perhaps.
Merge pull request #656 from aledsage/fix/rebind-NginxController

Fix memento/rebind of AbstractController