hammer2 - Revamp snapshot code

hammer2 - Revamp snapshot code

* Change the snapshot code to not use chain_duplicate().  Instead
  get the thing working by flushing the original sub-tree, creating
  a new directory under the super-root, and then simply copying the
  blockset.

  The previous code called chain_duplicate() to create the snapshot
  but the coding still required a flush.  Doing it without a flush
  is possible for a read-only snapshot but very difficult otherwise.

* This will simplify coding down the line.