Fix memento/rebind of AbstractController (0.5)

Cherry-picked from master into 0.5 (see https://github.com/brooklyncentral/brooklyn/pull/656)

- 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.