pdp: deletion in pdp proving task is too eager

[Kubuxu]

https://github.com/filecoin-project/curio/blob/pdpv0/tasks/pdp/task_prove.go#L721-L728

select piece_id, pdp_pieceref from pdp_data_set_pieces WHERE data_set = 19
+----------+--------------+
| piece_id | pdp_pieceref |
|----------+--------------|
| 0        | 1601         |
| 1        | 1601         |
+----------+--------------+
select * from parked_piece_refs where ref_id = 1601
+--------+----------+----------+--------------+-----------+
| ref_id | piece_id | data_url | data_headers | long_term |
|--------+----------+----------+--------------+-----------|
| 1601   | 701      | <null>   | {}           | True      |
+--------+----------+----------+--------------+-----------+

single ref_id can be used by multiple pdp pieces
We should be using a refcount which is contained in pdp_piecerefs