hammer2 - Cluster API cleanup

hammer2 - Cluster API cleanup

* Simplify hammer2_cluster_copy() to reduce ref count confusion.  It now
  always refs the underlying chains and the cluster.  The follow-up
  hammer2_cluster_lock() op can then be called with HAMMER2_RESOLVE_NOREF
  to avoid double-refs on the cluster and its underlying chains.

* Simplify hammer2_inode_lock_ex() and hammer2_inode_lock_sh().  These
  functions now use hammer2_cluster_lock() instead of rolling their own.

* Focus is now never set until a cluster is locked.  However, continue
  to cache a default focus in the hammer2_inode's embedded cluster.

* Remove some dead code.