DATAGRAPH-433: Begin fixing of how  uniquely indexed properties are handled
DATAGRAPH-433 Handle Uniqueness for Label based Entities

* added merge to Neo4jTemplate, Database
* lazy determination of the index-label-name
* initialize storedEntityType early on, update with hierarchy information later
* label handling (correctly add additional labels to the node entity on create)
* uniqueness handling (choose merge in case of unique properties for creation)
* collecting of labels of a Neo4jPersistentEntity as getAllLabels()
DATAGRAPH-433 Support for Label Based Unique Entity Creation