hammer2 - slave sync adjustments, doc update

hammer2 - slave sync adjustments, doc update

* Fix the hammer2_cluster*() API, do not multiply-ref underlying chains
  when a cluster is multiply-ref'd.  Fixes issues when underlying chain
  elements of the cluster are replaced during operations.

* Fix frontend writing issues when multiple slaves are present.

* Adjust documentation a bit.  Get rid of the 256-way split description
  for copyid use, we aren't going to do things that way so the size for
  each physical volume can be up to ~2^64 (maybe ~2^63 to be safe).

* Add a HAMMER2_LOOKUP_NODIRECT flag to prevent recursively returning
  the inode and remove the broken (cluster == cparent) test in the slave-sync
  code.  Doh, cluster structures are independently allocated, test was
  broken.

* Fix several other slave-sync issues when multiple slaves are present.
  (there are still some issues remaining).

* Fix accounting for the number of mounts using a physical device
  (hmp), rename pmp_count to mount_count for clarity.