Support for neo4j backend? [#2286293]

Problem/Motivation

Neo4j is an open source graph database.
See: http://www.neo4j.org/

Certain graph problems can be solved by such a database efficiently, while being (almost) impossible for traditional databases. In concrete, it can search for 2nd, 3rd, nth degree of relationships in a social network. Or it can tell you through what relationships (over n hops) two objects are connected. It's the kind of technology to find the fastest road / route between two points.

Itarato started to build a neo4j connector / integration and we see some duplicated problem solving.
https://drupal.org/project/neo4j_connector

Proposed resolution

Use Search API to index into neo4j. Write a backend that allows to persist into it.

We are unsure yet, how much extra configuration we need, in concrete, there are some different aspects to configure.

Also i'm not sure, if the search abstraction would work at all, or we we would completely drop that part and provide a native query interface. Thus, using Search API only to build the index.