RE: libpq debug log

Hi,

I created a new version patch. Please find attached my patch.

Changes:
Since v2 patch
I forgot to write the detail of v2 patch changes. So I write these.
- Fixed the " Start receiving message from backend:" message location. Because I found that message located at outside
of"retry4". 
 
- Changed location where output "start :" / "end :" messages and timestamp.　The reason of the change is that v1 patch
didnot correspond to Asynchronous Command Processing.
 
- Added documentation
- Added documentation how to check mistake of logdir and/or logsize. (Based on review comment of Jim san's)
Since v3 patch
- Fixed my mistake on fe-connect.c. Time and message were output at the place where does not output in originally
PQtrace().These messages are needed only new trace log. So I fixed it.
 
- Fixed two points so that new trace log overlaps all places in PQtrace(). (Based on review comment of Jacob san's)

TODO:
I will add the feature called "logging level" on next version patch. I will attach it as soon as possible.

I'm marking it as "Needs review".

Regards,
Aya Iwata