In FSX, replace txn / rev dual interfaces in index and cached_data code
with the new unified change set based ID part structure.

This is part of a series of patches switching everything over to the
new change set number paradigm.

* subversion/libsvn_fs_x/index.h
  (svn_fs_x__item_offset): Replace the txn / rev+item dual interface
                           parameters with a single change set based
                           ID part struct reference.  Also remove FSFS
                           references from the docstring.

* subversion/libsvn_fs_x/index.c
  (svn_fs_x__item_offset): Extract rev num / txn id internally as needed. 

* subversion/libsvn_fs_x/verify.c
  (compare_l2p_to_p2l_index,
   compare_p2l_to_l2p_index): Update index API calls. 

* subversion/libsvn_fs_x/cached_data.c
  (dgb__log_access): Combine revision+item_index into a id part parameter.
                     Also, remove FSFS-related code and docstring sections.
  (open_and_seek_revision): Combine rev+item to a single id part parameter
                            and update index API call.
  (open_and_seek_transaction): Update index API call.
  (open_and_seek_representation): Update function call.
  (get_node_revision_body,
   svn_fs_x__get_node_revision,
   svn_fs_x__get_mergeinfo_count): Update local function and index API calls. 
  (rep_state_t): Combine revision + item index members into an id part.
  (create_rep_state_body): Update struct initialization and use the new
                           struct member wherever required by function and
                           API changes.
  (get_window_key): Update struct user.
  (auto_set_start_offset, 
   read_delta_window,
   read_container_window, 
   get_combined_window): Update struct access and use the new struct member
                         wherever required by function and API changes.
  (init_rep_state): Update struct initialization.
  (svn_fs_x__get_changes): Update function and index API calls.
  (block_read): Update local function as well as index API calls and
                slightly simplify internal code.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568748 13f79535-47bb-0310-9956-ffa450edef68