pgsql: Switch pg_promote to be parallel-safe

Switch pg_promote to be parallel-safe

pg_promote uses nothing relying on a global state, so it is fine to mark
it as parallel-safe, conclusion based on a detailed analysis from Robert
Haas. This also fixes an inconsistency where pg_proc.dat missed to mark
the function with its previous value for proparallel, update which does
not matter now as the default is used.

Based on a discussion between multiple folks: Laurenz Albe, Robert Haas,
Amit Kapila, Tom Lane and myself.

Discussion: https://postgr.es/m/20181029082530.GL14242@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8f045e242b2308473f5de532590454294e3d5683

Modified Files
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src/backend/catalog/system_views.sql | 2 +-
src/include/catalog/catversion.h | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)