pgsql: Mop-up for parallel degree-ectomy.

Mop-up for parallel degree-ectomy.

Fix a couple of overlooked uses of "degree" terminology.  Make the parallel
worker count selection logic in create_plain_partial_paths more robust (in
particular, it failed with max_parallel_workers_per_gather set to zero).

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e4158319f34ceb2c760761dc93d262513235c344

Modified Files
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src/backend/optimizer/path/allpaths.c | 40 ++++++++++++++++-------------------
src/backend/optimizer/util/plancat.c  |  4 ++--
src/include/utils/rel.h               | 11 +++++-----
3 files changed, 26 insertions(+), 29 deletions(-)