Support multi-level partitioning

Citus currently only supports a single level of partitioning, but many applications require multiple levels (e.g. date + data source).

```sql
create table grand_parent (x int, t timestamptz) partition by range (t);
create table parent (x int, t timestamptz) partition by list (x);
alter table grand_parent attach partition parent for values from ('2000-01-01') to ('2010-01-01');
select create_distributed_table('grand_parent','x');
ERROR: distributing multi-level partitioned tables is not supported
DETAIL: Relation "parent" is partitioned table itself and it is also partition of relation "grand_parent".
```