## Description
When the `product.template.attribute.value` (PTAV) are edited and the respective `product.template.attribute` (PTA) variant creation config is set to `Instantly`, all existing variants for that template are unlinked and all the combinations possible of variants are recreated. This can be slow, specially for templates that already have a lot of variants.

## Patch
Main bottleneck is the hot loop in `product.product:unlink`, rewriting it a bit to pre-compute what is possible, before looping.
- Lift `exists` check outside the loop, it does 1 query.
- Batch the `search` in the main loop body of `product.product`'s `unlink` with a `_read_group`.
- Replace usage of `recordset`'s `union` with `set`'s `add`, due to the loop, complexity goes from `O(n^2)` -> `O(n)` for `n := len(self)`.

## Benchmark
On a DB with 1.2M `product.product`, adding a new PTAV to a `product. template` with 39k+ variants takes:

|         | Before             | After  |
|---------|--------------------|--------|
| Timings | timeout (ETC 3h+)* | 2.2min |

\* - ETC was calculated by extrapolating the search query in the loop, which was taking ~300ms, times the number of variants to be unlinked which was 39k+ variants.

## Reference
opw-4383294

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