Mark immutable functions in information schema as parallel safe

Mark immutable functions in information schema as parallel safe

Also add opr_sanity check that all preloaded immutable functions are
parallel safe.  (Per discussion, this does not necessarily have to be
true for all possible such functions, but deviations would be unlikely
enough that maintaining such a test is reasonable.)

Reported-by: David Rowley <david.rowley@2ndquadrant.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>