MB-62182: Avoid re-training vector indexes during merge

[Thejas-bhat]

• The main purpose of this PR is to avoid unnecessary re-training of the vector indexes during merge process.
• Going by the numbers, we need roughly 156K vectors for a 1M dataset ((min_num_vectors_per_centroid) * num_centroids = 39 * 4 * sqrt(1M)) as per recommendation
• The data ingestion is now split into 2 phases - the first phase involves creating a centroid index using the Train() API and the bolt is recorded with the progress in terms of samples trained upon. The second phase is just the normal indexing of data using the Batch() or the Index() APIs.
• Later on, when the vector indexes are getting merged the merger will use the centroid index to merge the inverted lists (centroids) in a block-wise fashion without reconstructing the layout.
• The feature can be enabled by passing a "vector_index_fast_merge": "true" key-value pair as part of kvconfig while creating/opening the index.

[abhinavdangeti]

@Thejas-bhat is this ready for review? Looks like it needs a rebase.

[coveralls]

coverage: 52.616% (-0.08%) from 52.693%
when pulling 9d87598 on fastmerge
into 6b72a24 on master.

[abhinavdangeti]

@Thejas-bhat have you verified unit tests to work under -tags=vectors?

[Thejas-bhat]

@Thejas-bhat have you verified unit tests to work under -tags=vectors?

yeah it passes on my local system

...
?   	github.com/blevesearch/bleve/v2/search/highlight/highlighter/ansi	[no test files]
?   	github.com/blevesearch/bleve/v2/search/highlight/highlighter/html	[no test files]
ok  	github.com/blevesearch/bleve/v2/search/highlight/highlighter/simple	(cached)
ok  	github.com/blevesearch/bleve/v2/search/query	(cached)
ok  	github.com/blevesearch/bleve/v2/search/scorer	(cached)
ok  	github.com/blevesearch/bleve/v2/search/searcher	5.670s
?   	github.com/blevesearch/bleve/v2/size	[no test files]
ok  	github.com/blevesearch/bleve/v2/test	20.841s
?   	github.com/blevesearch/bleve/v2/util	[no test files]