pg_dump: prepare attribute stats query.

pg_dump: prepare attribute stats query.

Follow precedent in pg_dump for preparing queries to improve
performance. Also, simplify the query by removing unnecessary joins.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reported-by: Andres Freund <andres@anarazel.de>
Co-authored-by: Corey Huinker <corey.huinker@gmail.com>
Co-authored-by: Jeff Davis <pgsql@j-davis.com>
Discussion: https://postgr.es/m/CADkLM=dRMC6t8gp9GVf6y6E_r5EChQjMAAh_vPyih_zMiq0zvA@mail.gmail.com