Better performing graphs for Mongo backend

[scaleupcto]

The current MongoGraph is very crude in the way it piggybacks on ExtendedGraph.

This causes excessive CPU as every single triple added via add_tripod_array is subject to regex via labelling.

Instead have a higher performance MongoGraph which uses the same internal representation as the database and moves the CPU load to extracting values out of the graph, rather than when putting them in.