Band-aid solution for problems with SubPlans used in hash join clauses,

Band-aid solution for problems with SubPlans used in hash join clauses,
per report from Andrew Holm-Hansen.  The difficulty arises from the fact
that the planner allowed a Hash node's hashkeys to share substructure
with the parent HashJoin node's hashclauses, plus some rather bizarre
choices about who initializes what during executor startup.  A cleaner
but more invasive solution is to not store hashkeys separately in the
plan tree at all, but let the HashJoin node deconstruct hashclauses
during executor startup.  I plan to fix it that way in HEAD.