TTL for columns works slower than CLEAR COLUMN

[dancing-elf]

Right now TTL for columns implemented as merges.
Maybe it is required for common cases, but we only want to drop data in column by partition. Our TTL expression is aligned with the partition expression.
Merges are pain for our tables, they need up to 10-20 hours to finish. And it's hurt queries timings.

As I understand in cases like above, TTL can be implemented as mutation, like CLEAR COLUMN.

[den-crane]

duplicate #26328