Use strtoi64() in pgbench, replacing its open-coded implementation

Here's a small patch to replace the int64 parsing code in pgbench with a 
call to strtoi64(). Makes it a little simpler.

Spotted this while grepping for all the different integer parsing 
functions we have. We could probably consolidate them some more, we 
still have quite a different integer-parsing routines in the backend and 
in the frontend. But this is one small, straightforward step in that 
direction.

- Heikki