refactoring - share str2*int64 functions

Hello Tom,

>> Yep, but ISTM that it is down to 32 bits,
>
> Only on 32-bit-long machines, which are a dwindling minority (except
> for Windows, which I don't really care about).
>
>> So the third short is now always 0. Hmmm. I'll propose another option over
>> the week-end.
>
> I suppose we could put pg_strtouint64 somewhere where pgbench can use it,
> but TBH I don't think it's worth the trouble.  The set of people using
> the --random-seed=int option at all is darn near empty, I suspect,
> and the documentation only says you can write an int there.

Although I agree it is not worth a lot of trouble, and even if I don't do 
Windows, I think it valuable that the behavior is the same on all 
platform. The attached match shares pg_str2*int64 functions between 
frontend and backend by moving them to "common/", which avoids some code 
duplication.

This is more refactoring, and it fixes the behavior change on 32 bit 
architectures.

-- 
Fabien.