Allow parallel DISTINCT

Allow parallel DISTINCT

We've supported parallel aggregation since e06a38965.  At the time, we
didn't quite get around to also adding parallel DISTINCT. So, let's do
that now.

This is implemented by introducing a two-phase DISTINCT.  Phase 1 is
performed on parallel workers, rows are made distinct there either by
hashing or by sort/unique.  The results from the parallel workers are
combined and the final distinct phase is performed serially to get rid of
any duplicate rows that appear due to combining rows for each of the
parallel workers.

Author: David Rowley
Reviewed-by: Zhihong Yu
Discussion: https://postgr.es/m/CAApHDvrjRxVKwQN0he79xS+9wyotFXL=RmoWqGGO2N45Farpgw@mail.gmail.com