pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c759395617765c5bc21db149cf8c3df52f41ccff

Modified Files
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src/backend/executor/nodeAppend.c     |  7 +++----
src/backend/optimizer/path/costsize.c | 16 ++++++++++++----
src/include/nodes/execnodes.h         |  1 -
3 files changed, 15 insertions(+), 9 deletions(-)