hammer2 - Start adding internal cluster API

hammer2 - Start adding internal cluster API

Initial addition of the cluster API.  H2 will be non-operational until this
is stabilized.  Adding the cluster API will require a few stages.  This first
stage is to add the API and make it work under degenerate (single-target)
conditions.

The hammer2_cluster structure collects and manages an array of up to
8 chains representing mirrors / cluster nodes / copies of the same point
in the topology.

* Add hammer2_cluster.c, and hammer2_cluster_t

* Replace nearly all high-level (vnops, vfsops) chain calls with
  cluster calls.