Rework the background laundering target after r304442.

Now that we launder less frequently and at a rate proportional to the
ratio of dirty to clean inactive pages, we want to process more pages during
each background laundering run. Launder a constant number of pages,
derived from the amount of system memory. Cap the rate at which pages are
laundered to 4MB/s and add a check to ensure we don't launder too much on
larger-memory systems if page daemon wakeups are infrequent and the number
of dirty pages is large.

Discussed with: alc