pgsql: pgbench: Synchronize client threads.

pgbench: Synchronize client threads.

Wait until all pgbench threads are connected before benchmarking begins.
This fixes a problem where some connections could take a very long time
to be established because of lock contention from earlier connections,
making results unstable and bogus with high connection counts.

Author: Andres Freund <andres(at)anarazel(dot)de>
Author: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
Reviewed-by: Marina Polyakova <m(dot)polyakova(at)postgrespro(dot)ru>
Reviewed-by: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: David Rowley <dgrowleyml(at)gmail(dot)com>
Discussion: https://postgr.es/m/20200227180100.zyvjwzcpiokfsqm2%40alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/aeb57af8e64000cc4288a7b8b8d7cf6040eae900

Modified Files
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src/bin/pgbench/pgbench.c | 42 +++++++++++++++++++++++++++++++++++++++++-
1 file changed, 41 insertions(+), 1 deletion(-)