Simplify the background laundering code and make it less aggressive.

• Remove the free page threshold for background laundering. It doesn't really add anything: we only start background laundering if the pagedaemon has been woken up recently, which means that the free page count dipped below the pagedaemon wakeup threshold.
• Reduce the maximum background laundering rate to 8MB/s. In HEAD, we will launder at most 32 * 16 pages * 2 pdwakeups/s = 4 MB/s during a first pass over the inactive queue.
• When laundering, use a target of
  
  0.1 * l(I)/l(L) * (free target - wakeup thresh)
  
  instead of
  
  0.5 * l(I)/l(L) * free target
  
  so that we effectively launder pages at a tenth the rate that they're reclaimed by the pagedaemon.

Reviewed by: alc