Split data store handling into classes and select alternatives using dependency injection

We have Neo4j and ArangoDB for the time being as graph data store options. Use Spring dependency injection to switch between alternatives.

Start by inspecting the current Neo4j and ArangoDB data-store handling and the Spring configuration that selects a store. Separate the handling into classes and make the active alternative selectable through dependency injection; done means both alternatives remain supported without one shared implementation handling them.

Written by the indexing model from the issue text.