PATCH: pgbench - aggregation of info written into log

Hi,

this patch adds support for aggregation of info written into the log.
Instead of info about each transaction, a summary for time intervals (with
custom length) is written into the log. All you need to do is add "-A
seconds", e.g.

  $ pgbench -T 3600 -A 10 -l db

which will produce log with 10-second summaries, containing interval start
timestamp, number of transactions, sum of latencies, sum of 2nd power of
latencies, min and max latency (it's done this way to allow handling of
multiple logs produced by "-j" option).

This patch is a bit less polished (and more complex) than the other
pgbench patch I've sent a while back, and I'm not sure how to handle the
Windows branch. That needs to be fixed during the commit fest.

kind regards
Tomas