FSX ID API cleanup: name things properly and remove unused functions.

* subversion/libsvn_fs_x/id.h
  (svn_fs_x__id_txn_used,
   svn_fs_x__id_txn_reset): Remove unused API function declarations.
  (svn_fs_x__id_rev_item): Rename to ...
  (svn_fs_x__id_noderev_id): ... this unified accessor.
  (svn_fs_x__id_rev_create): Rename to ...
  (svn_fs_x__id_create): ... this and name the noderev_id parameter as such.
                         This is now a unified rev / txn creator function.

* subversion/libsvn_fs_x/id.c
  (svn_fs_x__id_txn_used,
   svn_fs_x__id_txn_reset): Remove.
  (svn_fs_x__id_rev_item): Rename to ...
  (svn_fs_x__id_noderev_id): ... this.  Always return the id part because
                             this is now the unified accessor.
  (svn_fs_x__id_rev_create): Rename to ...
  (svn_fs_x__id_create): ... this and update parameter name.

* subversion/libsvn_fs_x/changes.c
  (append_change,
   svn_fs_x__changes_get_list,
   svn_fs_x__read_changes_container,
   svn_fs_x__changes_get_list_func): Simplify using the new unified ID
                                     accessor and constructor API.

* subversion/libsvn_fs_x/noderevs.c
  (binary_id_t): Name noderev_id properly.
  (store_id,
   get_id,
   svn_fs_x__write_noderevs_container,
   svn_fs_x__read_noderevs_container): Update following the struct member
                                       and ID API renames.

* subversion/libsvn_fs_x/cached_data.c
  (get_node_revision_body,
   svn_fs_x__get_node_revision, 
   svn_fs_x__get_mergeinfo_count): API callers follow the rename.

* subversion/libsvn_fs_x/pack.c
  (copy_node_to_temp): Ditto

* subversion/libsvn_fs_x/transaction.c
  (write_final_rev): Same.

* subversion/libsvn_fs_x/tree.c
  (x_node_relation): Same.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568794 13f79535-47bb-0310-9956-ffa450edef68