Code review for Parallel Append.

Code review for Parallel Append.

- Remove unnecessary #include mistakenly added in execnodes.h.
- Fix mistake in comment in choose_next_subplan_for_leader.
- Adjust row estimates in cost_append for a possibly-different
  parallel divisor.
- Clamp row estimates in cost_append after operations that may
  not produce integers.

Amit Kapila, with cosmetic adjustments by me.

Discussion: http://postgr.es/m/CAA4eK1+qcbeai3coPpRW=GFCzFeLUsuY4T-AKHqMjxpEGZBPQg@mail.gmail.com