Feature: Make database metadata pluggable, too.

[michael-simons]

Right now, we do the minimal approach, using standard neo4j procedures, that are available everywhere.
We also have the ability to define table to label mappings for the translator, though.

The latter could actually be expanded to general available metadata, so that one could define static metadata for a "table", such as movie (id INTEGER, length DECIMAL(12,2), title VARCHAR), or anything that fits current graph schema discussions, so that we can give a 100% exact answer wrt all metadata in specific cases.