Function build_datanode_plan 

pub(crate) async fn build_datanode_plan(
    table_metadata_manager: &TableMetadataManagerRef,
    node_id: DatanodeId,
) -> Result<DrainDatanodePlan>

Builds the drain plan of node_id.

Walks the datanode table index and reads the authoritative table routes, so the plan reflects the routes rather than the index. Each table route is split by role with region_distribution, which keys the regions by datanode id.