randomize vectors in the index when needed

randomizing vectors in the index is

• allows for new optimizations
• expensive (even with Eigen)
• not always necessary
• must be done for all vectors in the index, cannot be enabled mid-way

that is, we kind of want to do it, but don't want to do it if it's not needed (for Matryoshka or already randomized embeddings).

An idea: don't do it at first and let OPTIMIZE TABLE enable it