Mark advisory-lock functions as parallel restricted, not parallel unsafe.

Mark advisory-lock functions as parallel restricted, not parallel unsafe.

There seems no good reason not to allow a parallel leader to execute
these functions.  (The workers still can't, though.  Although the code
would work, any such lock would go away at worker exit, which is not
the documented behavior of advisory locks.)

Discussion: https://postgr.es/m/11847.1564496844@sss.pgh.pa.us