Flesh out first StorageFacet interface and impl

To support the initial raw content format, take an initial hack at the StorageFacet interface as roughly described here:
https://docs.sonatype.com/display/Nexus/Nexus+3+Storage+Facet#Nexus3StorageFacet-Facet

...and provide an initial implementation

This is expected to be reworked at some point. In order to provide something that basically works, the interface will provide direct access to OrientDB primitives (e.g. Vertices, Edges, and possibly the Graph) rather than trying to hide them behind other objects, where practical.