Neo4j stores: add a database schema version node to the database

[gordom6]

Our database is effectively write-once. We don't really need migrations and such. The middleware should only work with the most current database schema, and refuse to interoperate with an "old" database schema.

The Neo4j store in Scala should:

1. Refuse to write to a non-empty database i.e., there should only ever be one write transaction on a database
2. Check a non-empty database's schema version (node) against a version hard-coded in the Scala. If it's old, print a message and exit.