pgsql: Various pgbench enhancements.

Log Message:
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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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Modified Files:
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pgsql/contrib/pgbench:
README.pgbench (r1.15 -> r1.16)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgbench/README.pgbench.diff?r1=1.15&r2=1.16)
README.pgbench_jis (r1.16 -> r1.17)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgbench/README.pgbench_jis.diff?r1=1.16&r2=1.17)
pgbench.c (r1.62 -> r1.63)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgbench/pgbench.c.diff?r1=1.62&r2=1.63)