PATCH: pgbench / int64 instead of int for xact count

Hi,

I've been running a few longer pgbench tests (~week), and I've run into
this:

transaction type: SELECT only
scaling factor: 1250
query mode: simple
number of clients: 32
number of threads: 4
duration: 605000 s
number of transactions actually processed: -1785047856
latency average: -10.846 ms
tps = -2950.492090 (including connections establishing)
tps = -2950.492325 (excluding connections establishing)

The instance was doing ~10k tps for a week, which caused overflow of the
int counter, used to track number of transactions. Hence the negative
values.

I think we've reached the time when hardeare capable of doing this is
pretty common (SSDs, ...), so I think it's time to switch the counter to
int64.

Tomas