On the fsx-id branch: Begin to isolate our internal ID usage from 
FS API ID by storing only the noderev ID inside the DAG node.

We introduce two new DAG accessor functions, svn_fs_x__dag_get_fs_id
and the temporary svn_fs_x__dag_get_noderev_id.  The latter will
later be renamed to and replace svn_fs_x__dag_get_id.

To be able to update the DAG function implementations, we need two
new ID helpers and must switch other internal API functions to the
new noderev ID.

* subversion/libsvn_fs_x/id.h
  (svn_fs_x__id_part_reset,
   svn_fs_x__id_part_used): Declare new internal API functions.

* subversion/libsvn_fs_x/id.c
  (svn_fs_x__id_part_reset,
   svn_fs_x__id_part_used): Implement.

* subversion/libsvn_fs_x/cached_data.h
  (svn_fs_x__rev_get_root): Switch signature our internal ID type and
                            drop the result pool.  The caller now
                            provides the ID struct.

* subversion/libsvn_fs_x/cached_data.c
  (svn_fs_x__rev_get_root): Update Implementation.

* subversion/libsvn_fs_x/transaction.c
  (create_new_txn_noderev_from_rev): Switch to our internal ID type.
  (svn_fs_x__create_txn): Update caller.

* subversion/libsvn_fs_x/dag.h
  (svn_fs_x__dag_get_id): The node parameter is now, temporarily, mutable.
                          This will later be replaced by ...
  (svn_fs_x__dag_get_noderev_id): ... this new function.
  (svn_fs_x__dag_get_fs_id): New function.  To be used where we actually
                             want an FS API level ID.

* subversion/libsvn_fs_x/dag.c
  (dag_node_t): Switch members to our internal ID type.
  (svn_fs_x__dag_get_id): Reimplement to temporary backward compat.
                          Will eventually be replaced by ...
  (svn_fs_x__dag_get_noderev_id): ... this new code.
  (get_node_revision): Update and simplify.
  (svn_fs_x__dag_get_fs_id): Implement new function.
  (svn_fs_x__dag_check_mutable,
   svn_fs_x__dag_get_node,
   svn_fs_x__dag_get_revision,
   svn_fs_x__dag_set_proplist,
   svn_fs_x__dag_increment_mergeinfo_count,
   svn_fs_x__dag_set_has_mergeinfo,
   svn_fs_x__dag_revision_root,
   svn_fs_x__dag_clone_child,
   svn_fs_x__dag_dup): Update struct accessors and API callers.
  (svn_fs_x__dag_serialize,
   svn_fs_x__dag_deserialize): Update and simplify.
  (svn_fs_x__dag_update_ancestry): For now, use the ID provided in
                                   the noderev struct.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsx-id@1645655 13f79535-47bb-0310-9956-ffa450edef68