Allow total number of transactions in pgbench to exceed INT_MAX.

Change the total-transactions counters from int32 to int64 to accommodate
cases where we do more than 2^31 transactions during a run.  This patch
does not change the INT_MAX limit on explicit "-t" parameters, but it
does allow the product of the -t and -c parameters to exceed INT_MAX, or
allow a -T limit that is large enough that more than 2^31 transactions
can be completed.  While pgbench did not actually fail in such cases,
it did print an incorrect total-transactions count, and some of the
derived numbers such as TPS would have been wrong as well.

Tomas Vondra