pgbench - allow to specify scale as a size

After Karel Moppel piece on pgbench scale/size conversion, it occured to 
me that having this as an option would be nice.

https://www.cybertec-postgresql.com/en/a-formula-to-calculate-pgbench-scaling-factor-for-target-db-size/

Here is a attempt at extending --scale so that it can be given a size.

   pgbench -i --scale=124G ...

The approximated database size is also shown in the end-of-run summary.

-- 
Fabien.