backfill - minor performance enahcements

The gprof tool is showing most time is being consumed by the bit_test()
  function as called from the select plugin, which in turn was called
  by the backfill scheduler. These changes replace the for loop end-points.
  Previous logic tested for all possible nodes. The new logic identifes
  the first and last bit set in the node bitmap and uses those end-points
  instead. Node the logic to find the first and last bits set starts off
  with a word-based search (testing for a 64-bit zero value rather than
  testing each individual bit). The net result is a small performance
  improvement.
bug 2588