Dump labels in reproducible order

Dump labels in reproducible order

In pg_get_propgraphdef(), sort the labels before writing out, for a
consistent dump order.  Also, since we now have a list, we can get rid
of the separate table scan to get the count.

Co-authored-by: Peter Eisentraut <peter@eisentraut.org>
Co-authored-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Co-authored-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/a855795d-e697-4fa5-8698-d20122126567@eisentraut.org