Function plan_worker_reads 

fn plan_worker_reads(
    config: &RawSourceCreationConfig,
    tables: &BTreeMap<MySqlTableName, Vec<SourceOutputInfo>>,
    pk_bounds: &BTreeMap<MySqlTableName, Option<PkBoundaries>>,
) -> BTreeMap<MySqlTableName, ReadPlan>

Returns the set of full tables/sections of tables to read.