Allow pgbench to use a scale larger than 21474.

Beyond 21474, the number of accounts exceed the range for int4. Change the
initialization code to use bigint for account id columns when scale is large
enough, and switch to using int64s for the variables in pgbench code. The
threshold where we switch to bigints is set at 20000, because that's easier
to remember and document than 21474, and ensures that there is some headroom
when int4s are used.

Greg Smith, with various changes by Euler Taveira de Oliveira, Gurjeet
Singh and Satoshi Nagayasu.