pgsql: Change SeqScan node to contain Scan node

Change SeqScan node to contain Scan node

This makes the structure of all Scan-derived nodes the same,
independent of whether they have additional fields.

Discussion: /messages/by-id/c1097590-a6a4-486a-64b1-e1f9cc0533ce@enterprisedb.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2226b4189bb4ccfcc53917a8695d24e91ff2f950

Modified Files
--------------
src/backend/executor/nodeSeqscan.c | 4 ++--
src/backend/nodes/readfuncs.c | 2 +-
src/backend/optimizer/plan/createplan.c | 6 +++---
src/backend/optimizer/plan/setrefs.c | 10 +++++-----
src/include/nodes/plannodes.h | 5 ++++-
5 files changed, 15 insertions(+), 12 deletions(-)