This reverts commit 7204f359,
thus restoring 66c0185a (Allow planner to use Merge Append to
efficiently implement UNION) as well as the follow-on commits
d5d2205c, 3b1a7eb2, 7487044d.

Per further discussion on pgsql-release, we wish to ship beta1 with
this feature, and patch the bug that was found just before wrap,
rather than shipping beta1 with the feature reverted.