pgsql: Add libpq pipeline mode support to pgbench

Add libpq pipeline mode support to pgbench

New metacommands \startpipeline and \endpipeline allow the user to run
queries in libpq pipeline mode.

Author: Daniel Vérité <daniel@manitou-mail.org>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/b4e34135-2bd9-4b8a-94ca-27d760da26d7@manitou-mail.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9aa491abbf07ca8385a429385be8d68517384fdf

Modified Files
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doc/src/sgml/ref/pgbench.sgml                |  22 +++++
src/bin/pgbench/pgbench.c                    | 131 ++++++++++++++++++++++++---
src/bin/pgbench/t/001_pgbench_with_server.pl |  79 +++++++++++++++-
3 files changed, 216 insertions(+), 16 deletions(-)