pgbench transaction timestamps

This patch changes the way pgbench outputs its latency log files so that
every transaction gets a timestamp and notes which transaction type was
executed. It's a one-line change that just dumps some additional
information that was already sitting in that area of code. I also made a
couple of documentation corrections and clarifications on some of the more
confusing features of pgbench.

It's straightforward to parse log files in this format to analyze what
happened during the test at a higher level than was possible with the
original format. You can find some rough sample code to convert this
latency format into CVS files and then into graphs at
http://www.westnet.com/~gsmith/content/postgresql/pgbench.htm which I'll
be expanding on once I get all my little patches sent in here.

If you recall the earlier version of this patch I submitted, it added a
cleanup feature that did a vacuum and checkpoint after the test was
finished and reported two TPS results. The idea was to quantify how much
of a hit the eventual table maintenance required to clean up after the
test would take. While those things do influence results and cause some
of the run-to-run variation in TPS (checkpoints are particularly visible
in the graphs), after further testing I concluded running a VACUUM VERBOSE
and CHECKPOINT in a script afterwards and analyzing the results was more
useful than integrating something into pgbench itself.

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* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD