graph index search: heuristical edge choice

basic graph search algorithm takes all neighbors of a top node and adds them all back to the todo queue, sorted by the distance to the query. this implies many expensive distance calculations.

there are heuristics that precalculate directions to neighbors and first use neighbors that are in the same direction as the query. HCNNG does that.

Their primary goal is to save on distance calculations — so we should do that only if profiling will show that distance calculations take a sizable amount of time as compared to the total query duration.