bug: hardcoded cluster on async migrations

[gempir]

Migrations with mode=async error with Requested cluster 'cluster' not found even when clickhouse.cluster is set (e.g. moebel_cluster) and the SQL uses ON CLUSTER 'moebel_cluster'.

I think the problem are the these points here using the wrong cluster.

https://github.com/obsessiondb/chkit/blob/main/packages/clickhouse/src/index.ts#L769
https://github.com/obsessiondb/chkit/blob/main/packages/clickhouse/src/index.ts#L795

Same in https://github.com/obsessiondb/chkit/blob/main/packages/plugin-backfill/src/async-backfill.ts#L152