Stabalize the clump paths by clamping the number of pending sets and

Stabalize the clump paths by clamping the number of pending sets and
cleaning up the sleeping logic.  Also tie in ll_readpage.   Now to start
profiling and tuning this..
- record the rw mode in llwp so completion knows what to do with the pages
- introduce a simple little 'page list' data structure that uses page->private
  to chain pages off llis in fifo order
- have both read and write plists in the lli
- get rid of the old unused work_pending member
- always put the lli back on the list in _give_plist so that we don't
  lose llis
- introduce a 'new_arrival' member so lliod can only come out of l_wait_event
  when new material arrives
- pass rc into llwp completion so we can make use of it someday
- move the concurrent set checks into the set building loop, duh
- add some debugging here and there