hammer2 - structuralize the cluster's chain array

hammer2 - structuralize the cluster's chain array

* Structuralize hammer2_cluster's chain array, implementing a new
  sub-structure called hammer2_cluster_item.

  This structure represents one of N elements in a cluster node representing
  the same location at each target.  It will also be used to track cache
  coherency state, async locks, and other things.