Various pgbench enhancements. Patch contributed by ITAGAKI Takahiro.

Also tweak README.pgbench/README.pgbench_jis:
  Remove history after pgbench was added to PostgreSQL contrib module.
  Those info was not only redundant since it has already been in CVS
  log, but also incomplete.
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The attached is a patch to optimize contrib/pgbench using new 8.3 features.

- Use DROP IF EXISTS to suppress errors for initial loadings.
- Use a combination of TRUNCATE and COPY to reduce WAL on creating
  the accounts table.

Also, there are some cosmetic changes.

- Change the output of -v option from "starting full vacuum..."
  to "starting vacuum accounts..." in reflection of the fact.
- Shape duplicated error checks into executeStatement().


There is a big performance win in "COPY with no WAL" feature.
Thanks for the efforts!
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