Bulk score hnsw diversity check

[benwtrent]

This adds bulk scoring to diversity check. While this means that diversity check cannot exit super early (e.g. if it only needs to check 2 docs), I continually see diversity check as being the most expensive part of HNSW graph merging.

This tells me that typically, it isn't just one doc that is checked.

I ran 1M 768 cohere, force-merging with 4 threads.

baseline: 128.01
candidate: 92.15