EvalPlanQual was thoroughly broken for concurrent update/delete on inheritance

EvalPlanQual was thoroughly broken for concurrent update/delete on inheritance
trees (mostly my fault).  Repair.  Also fix long-standing bug in ExecReplace:
after recomputing a concurrently updated tuple, we must recheck constraints.
Make EvalPlanQual leak memory with somewhat less enthusiasm than before,
although plugging leaks fully will require more changes than I care to risk
in a dot-release.