BGWriter latch, power saving

As part of the ongoing effort to reduce wake-ups when idle/power
consumption, the attached patch modifies the background writer to
hibernate in ten second bursts once the bgwriter laps the clock sweep.
It's fairly well commented, so a description of how it works here
would probably be redundant. The most controversial aspect of this
patch is the fact that it adds a SetLatch() call to MarkBufferDirty(),
though I went to some lengths to ensure that that call will very
probably find the latch already set when it actually matters, so it
only checks a single flag.

Thoughts? I can produce benchmarks, if that helps, but I think it's
fairly unlikely that the patch introduces a measurable performance
regression.

-- 
Peter Geoghegan       http://www.2ndQuadrant.com/
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