Get property w/o fetching the object first (#669).

The `DeepaMehtaService` has 2 new methods:
{{{
Object getProperty(long id, String propUri)
boolean hasProperty(long id, String propUri)
}}}
They access a Topic/Association property without obtaining the Topic/Association object in advance.
The `id` is a Topic ID or an Association ID.

BREAKING CHANGES

The `DeepaMehtaStorage` interface has changed, so storage layer implementations must be adapted.
The Neo4j based implementation is adapted.

See #669.