Memory optimisation: deduplicate identical `AdjacentNodes.offsets` arrays

[mpollmeier]

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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.