XML serialisation: avoid error if try to serialize entity

- If entity/location/policy sneaks through to be serialized
  (e.g. through config key of LocationConfigKeys.CALLER_CONTEXT
  pointing at Entity)...
- Don't blow up: instead let it deserialize as null, which is better
  than failing to serialize+deserialize everything about the entity.
- Logs at WARN for first time encountered, and debug subsequently.
XML persist: fix serializing MutableSet
Adds Identifiable interface

- implemented by Entity, Location and EntityAdjunct
Merge pull request #1017 from aledsage/fix/failing-rebind-tests-06x

Fix failing rebind tests (0.6.x)