Handling a case to avoid bringing faiss index to memory unnecessarily

[Thejas-bhat]

• In update/delete heavy scenario where we short circuit the merge path, we end up reading a faiss index to the memory unnecessarily and it can potentially stay in memory till the reconstruction of all valid vectors is complete (which is not ideal).
• Also refactoring the vecIndexes []*faiss.IndexImpl construct to indexes []*vecIndexInfo just so we don't have to read info from two different slices while reconstructing vectors.