Add option for HNSW indexing of vectors

[kellrott]

Indexing method: https://arxiv.org/abs/1603.09320
Example code: https://github.com/Bithack/go-hnsw

One idea would be to refactor existing indexing code used by the KV graph code, so that it can be layered onto other drivers (mongo, sql, elastic) as middleware and index node/edges as they are inserted into the graph. Allowing vector searching to be implemented on all drivers.