Switch to linear search below a threshold

[vadixidav]

Linear search is generally faster than HNSW on a given computer below a certain threshold value. We should create a configurable size at which the HNSW transitions from linear search internally to its own search. The default for that transition point should be around where benchmarks say it crosses over. Benchmarks already exist that show the user the tradeoff point.

[vadixidav]

Linear search should actually be used in every layer until a layer has more features than it would be worth to do with linear.