Persist EntityRef in xml-serialization (WIP)

- Instead of replacing the entity with a string, and storing which
  fields/config/attributes this had been done to, we persist an
  EntityRef or LocationRef. On deserialisation this can be replaced
  with an EntityProxy.
- TODO: at deserialisation of the mementos, we don't have all the
  entities yet so can't crate all the proxies. E.g. if the first file
  references the second, then we can't deserialise the first's EntityRef
  into an EntityProxy.
AstyanaxSample to use different columnFamilyNames
Remove use of MementoTransformer (in persistence)
Persist completed attributeWhenReady config

- currently it's belt and braces (!)
-- MementoGenerators swaps the task for the completed val or null
-- XmlMementoSerializer has a converter for Task
Expand machine-rebind testing
Cleanup XmlMementoSerializer
Testing SshMachineLocation persistence

- and improve logging