pgsql: Add a 'parallel_degree' reloption.

Add a 'parallel_degree' reloption.

The code that estimates what parallel degree should be uesd for the
scan of a relation is currently rather stupid, so add a parallel_degree
reloption that can be used to override the planner's rather limited
judgement.

Julien Rouhaud, reviewed by David Rowley, James Sewell, Amit Kapila,
and me. Some further hacking by me.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/25fe8b5f1ac93c3ec01519854e4f554b2e57a926

Modified Files
--------------
doc/src/sgml/ref/create_table.sgml | 13 +++++++
src/backend/access/common/reloptions.c | 17 +++++++--
src/backend/optimizer/path/allpaths.c | 64 +++++++++++++++++++++++-----------
src/backend/optimizer/util/plancat.c | 3 ++
src/backend/optimizer/util/relnode.c | 1 +
src/bin/psql/tab-complete.c | 1 +
src/include/nodes/relation.h | 1 +
src/include/utils/rel.h | 9 +++++
8 files changed, 86 insertions(+), 23 deletions(-)