Memory optimisation: deduplicate identical AdjacentNodes.offsets arrays

[mpollmeier]

In large graphs we're wasting ~8% of the heap for duplicate arrays in AdjacentNodes.offsets.
That array is only ever mutated in AdjacentNodes.setOffset in a very constraint and straighforward way, so we could easily optimise it.

E.g. we could have a global TIntObjectMap as a cache that maps from the checksum of a given offset array to the array.