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