pgsql: Add "G" (server-side data generation) as an initialization step

Add "G" (server-side data generation) as an initialization step in pgbench.

This commit allows --init-steps option in pgbench to accept "G" character
meaning server-side data generation as an initialization step.
With "G", only limited queries are sent from pgbench client and
then data is actually generated in the server. This might make
the initialization phase faster if the bandwidth between pgbench client
and the server is low.

Author: Fabien Coelho
Reviewed-by: Anna Endo, Ibrar Ahmed, Fujii Masao
Discussion: https://postgr.es/m/alpine.DEB.2.21.1904061826420.3678@lancre

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a386942bd29b0ef0c9df061392659880d22cdf43

Modified Files
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doc/src/sgml/ref/pgbench.sgml | 30 ++++++++-
src/bin/pgbench/pgbench.c | 98 ++++++++++++++++++++++------
src/bin/pgbench/t/001_pgbench_with_server.pl | 4 +-
src/bin/pgbench/t/002_pgbench_no_server.pl | 2 +-
4 files changed, 108 insertions(+), 26 deletions(-)