pgbench: introduce a new automatic variable 'client_number'

Please find attached a patch for pgbench that introduces a new
auto-variable 'client_number'. Following in the footsteps of 'scale'
auto-variable, this is not declared if the user has specified this variable
using -D switch.

Since 'clientid' is a very common name a user can use for their own
script's variable, I chose to call this auto-variable client_number; just
to avoid conflicts.

This variable can come in handy when you want to use a different expression
in a query depending on which client is executing it. An example custom
transaction is attached, where the UPDATE statement from any given client
always updates the same logical row.

Best regards,
--
Gurjeet Singh

http://gurjeet.singh.im/

EnterpriseDB Inc.