Attached is a new version of the bgwriter stats patch. Per previous
discussion, now uses the stats system only. Introduces a new stats message
for bgwriter, and also introduces a "global stats" part of the stats file
for statistics not bound to a database or table.
I've included a couple of more counters per ideas from Greg Smith in his
logging patch.
Unless there are objections, I'll go ahead and apply this version of the
patch (once the documentation is written, of course).
//Magnus
