hammer2 - bulkfree work, rip-up cluster sync.

hammer2 - bulkfree work, rip-up cluster sync.

* bulkfree no longer attempts to flush.  Instead it deals with races against
  live by refusing to free blocks in L1 freemap chains that have been modified
  since the last sync.  This is a temporary workaround.

* No longer propagate modify_tid during a flush.  modify_tid is now used
  as a localized but cluster-aware TID (whereas mirror_tid is only localized
  to a cluster node).

* Start work on adding an update_tid to the blockref.  This will ultimately
  be used by the cluster synchronization code instead of modify_tid.

* Adjust the DESIGN document for the new synchronization concept.