Create triple pattern indexes for all revisions

[mgns]

The index must be able to tell in which revision a particular triple pattern is contained.

For each triple in a revision compute a hash for:

• s p o
• s p _
• s _ o
• s _ _
• _ p o
• _ p _
• _ _ o

The hash points to all changesets containing respective triples.
To retrieve a particular triple pattern compute it's hash and retrieve all triples from the states.

!! Optimizations to apply !!