Unbreak Finalize HashAggregate over Partial HashAggregate.

Commit 5dfc1981 introduced the use
of a new type of hash table with linear reprobing for hash aggregates.
Such a hash table behaves very poorly if keys are inserted in hash
order, which does in fact happen in the case where a query use a
Finalize HashAggregate node fed (via Gather) by a Partial
HashAggregate node.  In fact, queries with this type of plan tend
to run effectively forever.

Fix that by seeding the hash value differently in each worker
(and in the leader, if it participates).

Andres Freund and Robert Haas