Find out if NBHM is better or worse than ConcurrentHashMap in OnHeapGraphIndex about jvector HOT 2 CLOSED

@jbellis I did some synthetic testing in the cases I'd expect to stress the difference. Workloads included concurrency levels of 4/8/16/32/64 threads with mixed read-write workloads profiles consisting of 12.5% writes, 25% writes, and 50% writes. In addition, I also tested concurrency levels of 8/16/32 prebuilding the graph (all writes) and searching random vectors (all reads).

Profiles confirmed this set up did cause data structure operations to dominate search/indexing, as desired. In all cases, the differences between CHM and NBHM were minimal and within run-to-run variation. Given this, I think there's no reason to advocate for a change at this time. We could revisit this down the road, but I think there are plenty of higher payoff changes right now.

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Deps slimmed in #49

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