hammer2 - Sync thread work

hammer2 - Sync thread work

* Initial syncthread coding, still many issues to resolve, particularly
  with the handling of mirror_tid, but it was able to successfully
  synchronized a MASTER to a SLAVE.

* Add more features to the hammer2_cluster*() API and structures and fix
  a couple of bugs in hammer2_cluster_lookup() and hammer2_cluster_next().

* Add hammer2_cluster_next_single_chain() for advancing one element of
  a cluster structure.  Used to advance past a chain which the sync code
  must delete on a slave.

* Fix a bug in hammer2_chain_lookup() when HAMMER2_LOOKUP_NOLOCK is used.