Skip updating search when changed DB columns do not affect results

[NyanKiyoshi]

When mutations such as productUpdate() are invoked, the product's search vector
should only be updated if it's actually changing columns that are saved as part of the
search vector, such as: product name, description, attributes, etc.¹

For example, product metadata are not search indexed and thus shouldn't lead to
a product re-indexing.

Every mutation that causes to update search indexes should be reviewed, and optimized.

Solving this issue would reduce resources needed and costs.

Footnotes

1. https://github.com/saleor/saleor/blob/b17f360e4e52d12be1f9a3828ff0211adf7001d1/saleor/product/search.py#L63-L78 ↩