Allow parallel DISTINCT

Back in March 2016, e06a38965 added support for parallel aggregation.
IIRC, because it was fairly late in the release cycle, I dropped
parallel DISTINCT to reduce the scope a little. It's been on my list
of things to fix since then. I just didn't get around to it until
today.

The patch is just some plumbing work to connect all the correct paths
up to make it work. It's all fairly trivial.

I thought about refactoring things a bit more to get rid of the
additional calls to grouping_is_sortable() and grouping_is_hashable(),
but I just don't think it's worth making the code ugly for. We'll
only call them again if we're considering a parallel plan, in which
case it's most likely not a trivial query. Those functions are pretty
cheap anyway.

I understand that there's another patch in the September commitfest
that does some stuff with Parallel DISTINCT, but that goes about
things a completely different way by creating multiple queues to
distribute values by hash. I don't think there's any overlap here.
We'd likely want to still have the planner consider both methods if we
get that patch sometime.

David