pgbench: Correct comparison ops for reporting

Signed-off-by: Mel Gorman <mgorman@suse.de>
pgbench: Reinit database for every thread count test

Signed-off-by: Mel Gorman <mgorman@suse.de>
pgbench: Configure for autovacumn

Signed-off-by: Mel Gorman <mgorman@suse.de>
pgbench: Automatically select suitable transactions count if configured

Signed-off-by: Mel Gorman <mgorman@suse.de>