Accidentally parallel unsafe functions

Hi,

I am currently looking into adding the correct parallel options to all
functions in the extensions and I noticed that some built-in functions
seems to have been marked as unsafe by accident. The main culprit is
system_views.sql which redefines these functions and removes the
parallel safe flag.

I think this counts as a 9.6 bug unlike my work on adding the flags to
all extensions which is for 9.7.

I have attached a patch which marks them and all conversion functions as
parallel safe. I also added the flag to ts_debug() when I was already
editing system_views.sql, feel free to ignore that one if you like.

Affected functions:

- json_populate_record()
- json_populate_recordset()
- jsonb_insert()
- jsonb_set()
- make_interval()
- parse_ident()
- Loads of conversion functions

Andreas