pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6ee3b91bad269b8f1acee38607bd17923efd6e06

Modified Files
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src/bin/pg_dump/pg_backup.h |   1 +
src/bin/pg_dump/pg_dump.c   | 123 ++++++++++++++++++++------------------------
2 files changed, 58 insertions(+), 66 deletions(-)