Adj ivf postings list building

[benwtrent]

This adjusts the postings list building to be more idiomatic in how it access the vectors:

• First, all the vectors are read in order and quantized according to their centroid assignments
• Then we use those quantized values to read (potentially randomly) when building the clusters.

This reduces memory thrashing and improves indexing throughput at larger scale.

We should consider how we want to read these raw vectors during indexing to see if there are other gains to be had.

baseline:

index_name index_type num_docs index_time(ms) force_merge_time(ms) num_segments ------------------------------ ---------- -------- -------------- -------------------- ------------ cohere-wikipedia-docs-768d.vec ivf 4000000 345710 918228 0 

candidate:

index_name index_type num_docs index_time(ms) force_merge_time(ms) num_segments ------------------------------ ---------- -------- -------------- -------------------- ------------ cohere-wikipedia-docs-768d.vec ivf 4000000 294118 830919 0 

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[iverase]

I think we should wait until #129071 is resolved as this might change the way we write posting lists (not the format but the order).

[john-wagster]

lgtm