pgsql: pgbench: Add \syncpipeline

pgbench: Add \syncpipeline

This change adds a new meta-command called \syncpipeline to pgbench,
able to send a sync message without flushing using the new libpq
function PQsendPipelineSync().

This meta-command is available within a block made of \startpipeline and
\endpipeline.

Author: Anthonin Bonnefoy
Discussion: https://postgr.es/m/CAO6_XqpcNhW6LZHLF-2NpPzdTbyMm4-RVkr3+AP5cOKSm9hrWA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/94edfe250c6a200d2067b0debfe00b4122e9b11e

Modified Files
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doc/src/sgml/ref/pgbench.sgml                | 12 +++++++---
src/bin/pgbench/pgbench.c                    | 28 +++++++++++++++++++---
src/bin/pgbench/t/001_pgbench_with_server.pl | 36 ++++++++++++++++++++++++++++
3 files changed, 70 insertions(+), 6 deletions(-)