Seeds are dropped before recreated

**Is your feature request related to a problem? Please describe.**
Because seeds are dropped and then recreated, we get errors in the short period of downtime when seeds are used in materialized views. Also, if the recreation of the seed fails for some reason, the downtime lasts until seed is recreated.

**Describe the solution you'd like**
Use exchange instead of drop and create.

The issue names no file or test. Start by locating the seed drop/create lifecycle in the dbt-clickhouse adapter and review how ClickHouse exchange applies to seeds; done means materialized views avoid the drop window and a failed replacement does not leave the prior seed unavailable, with coverage for those cases.

Written by the indexing model from the issue text.