trace_checkpoint parameter patch

Hi all,

Here is a brand new patch to log a checkpointing load information
to tune the bgwriter parameter.

When you enable trace_checkpoint parameter and process
the checkpoint, the bgwriter logs the number of
flushed buffer pages and the elapsed time.

-----------------------------------------------------------
{61}snaga(at)artemis:~/pgsql-snapshot% ./bin/psql -c 'checkpoint' pgbench
LOG: CheckPoint: 1522/3072 buffer(s) flushed. CPU 0.03s/0.00u sec elapsed 1.57 sec
CHECKPOINT
{62}snaga(at)artemis:~/pgsql-snapshot%
-----------------------------------------------------------

I think this information is useful to configure
the bgwriter parameter to reduce the performance impact
of the checkpointing.

Any suggestion or comments?
--
NAGAYASU Satoshi <snaga(at)snaga(dot)org>