Make bgwriter sleep longer when it has no work to do, to save electricity.

To make it wake up promptly when activity starts again, backends nudge it
by setting a latch in MarkBufferDirty(). The latch is kept set while
bgwriter is active, so there is very little overhead from that when the
system is busy. It is only armed before going into longer sleep.

Peter Geoghegan, with some changes by me.