Core: eventually drop MehtaGraph API (#391).

The concept of a DM-independant MehtaGraph API is abolished. That intermediate storage layer is now dropped.
Instead DM storage implementations are coded to the DM model API directly.
The remaining "MehtaGraph"/"MG" wording in class names etc. will be dropped soon.

Neo4j traversal framework is not used anymore as it suffers from the super node performance problem.
Instead further Lucene indexes are deployed.

Not yet functional.

BREAKING CHANGES

Core: interface DeepaMehtaTransaction has moved.
Application developers must change their imports from
{{{
    import de.deepamehta.core.DeepaMehtaTransaction
}}}
to
{{{
    import de.deepamehta.core.storage.spi.DeepaMehtaTransaction
}}}

See ticket 391.