hammer2 - SPAN protocol work

hammer2 - SPAN protocol work

* Because we allow loops in the graph the loss of a feeder node can
  result in a tail-chasing loop of SPAN updates with an ever-growing
  distance parameter.

  To deal with this a spanning tree distance limit is required, beyond
  which no propagation occurs which terminates the chase.  The tail
  then catches up to the head and the node is finally removed from
  the spanning tree entirely.

  This fixes the propagation of spanning tree deletions e.g. when we
  umount a HAMMER2 PFS.

* Fix a state insertion bug.  A structure was being inserted into the
  red-black tree before the required fields were being initialized.
  Corrects a SPAN propagation fault.