Support delete operator

delete from t1 where a in (select * from t2)

->

delete from t1 where pk in (
  select pk, shard_key from t1 where a in (select * from t2)
)

- delete (t1)
  - motion (local segment / segment)            <- we would like to remove sharding key
                                                   from VT on resharding (need only pk and buckets)
    - projection (t1 primary key, sharding key)
      - selection (a in $1)
        - scan t1
$1:
- motion (full / segment / none)
  - projection (b)
    - scan t2