pgbench hard coded constants

pgbench has #defines for number of branches, tellers, and accounts. There
are used to populate the tables with -i, but when running actual benchmark
it has values separately hard-coded in the query metacommands. This patch
makes the metacommands obtain their values from the relevant #defines.

It has been tested to the extent that after changing the #define naccounts
downward, without the patch, after running both -i once and the benchmark
itself once leads to inconsistent results (select sum(abalance) from
pgbench_accounts does not equal sum(delta) from pgbench_history), while
with the patch they are equal.

Cheers,

Jeff