Improve planner's choices about when to use hashing vs sorting for DISTINCT.

Improve planner's choices about when to use hashing vs sorting for DISTINCT.

The previous coding missed a bet by sometimes picking the "sorted" path
from query_planner even though hashing would be preferable.  To fix, we have
to be willing to make the choice sooner.  This contorts things a little bit,
but I thought of a factorization that makes it not too awful.