pgbench: add --partitions and --partition-method options.

pgbench: add --partitions and --partition-method options.

These new options allow users to partition the pgbench_accounts table by
specifying the number of partitions and partitioning method.  The values
allowed for partitioning method are range and hash.

This feature allows users to measure the overhead of partitioning if any.

Author: Fabien COELHO
Reviewed-by: Amit Kapila, Amit Langote, Dilip Kumar, Asif Rehman, and
Alvaro Herrera
Discussion: https://postgr.es/m/alpine.DEB.2.21.1907230826190.7008@lancre