On the log-addressing branch:  Introduce the notion of "item_index" as a
generalized replacement for "offset" that works in both adressing modes.

This requires 3 sets of changes
- rename "offset" to "item_index" in structs and some local variables
- hange the FSFS ID API to say "item" instead of "offset"
- translate the item_index into actual file offsets before accessing
  the respective file

For the latter, we extend the existing svn_fs_fs__item_offset function
to take an optional TXN_ID parameter to make it suitable for general
offset translation.  The current implementation remains simple because
we don't really have logical addresses, yet.  Use that mapping API also
in those few places where we don't do so already.

* subversion/libsvn_fs_fs/id.h
  (svn_fs_fs__id_part_t): update commentary
  (svn_fs_fs__id_rev_offset,
   svn_fs_fs__id_offset): rename these to ...
  (svn_fs_fs__id_rev_item,
   svn_fs_fs__id_item): ... these, respectively
  (svn_fs_fs__id_rev_create): rename parameter

* subversion/libsvn_fs_fs/id.c
  (fs_fs__id_t): rename rev_offset member to rev_item
  (svn_fs_fs__id_rev_offset,
   svn_fs_fs__id_offset): rename these to ...
  (svn_fs_fs__id_rev_item,
   svn_fs_fs__id_item): ... these, respectively; update struct user
  (svn_fs_fs__id_rev,
   svn_fs_fs__id_unparse,
   svn_fs_fs__id_txn_create_root,
   svn_fs_fs__id_txn_create,
   svn_fs_fs__id_parse): update struct user
  (svn_fs_fs__id_rev_create): ditto; update signature

* subversion/libsvn_fs_fs/util.h
  (svn_fs_fs__item_offset): update signature; add TXN_ID parameter

* subversion/libsvn_fs_fs/util.c
  (svn_fs_fs__item_offset): update

* subversion/libsvn_fs_fs/fs.h
  (representation_cache_key_t,
   window_cache_key_t,
   fs_fs_data_t,
   representation_t): replace offsets with 64 bit item_index

* subversion/libsvn_fs_fs/low_level.h
  (svn_fs_fs__rep_header_t): rename base_offset to base_item_index

* subversion/libsvn_fs_fs/low_level.c
  (svn_fs_fs__parse_representation,
   svn_fs_fs__unparse_representation,
   svn_fs_fs__read_rep_header,
   svn_fs_fs__write_rep_header): update struct users

* subversion/libsvn_fs_fs/cached_data.c
  (dbg_log_access,
   open_and_seek_revision): update signature; struct usage and offset translation
  (open_and_seek_transaction): translate offset before accessing the file
  (open_and_seek_representation): update struct usage
  (get_node_revision_body,
   svn_fs_fs__get_node_revision): update API caller
  (rep_state_t): rename offset to item_index
  (auto_set_start_offset): update API caller
  (create_rep_state_body): update struct usage and offset translation
  (svn_fs_fs__rep_chain_length,
   get_window_key,
   build_rep_list,
   read_delta_window,
   svn_fs_fs__get_contents,
   svn_fs_fs__try_process_file_contents,
   svn_fs_fs__get_file_delta_stream,
   locate_dir_cache,
   svn_fs_fs__get_proplist): update struct usage

* subversion/libsvn_fs_fs/fs_fs.c
  (svn_fs_fs__noderev_same_rep_key): ditto

* subversion/libsvn_fs_fs/pack.c
  (compare_dir_entries_format6): update API caller

* subversion/libsvn_fs_fs/recovery.c
  (recover_find_max_ids,
   svn_fs_fs__find_max_ids): translate offsets before usage

* subversion/libsvn_fs_fs/rep-cache.c
  (svn_fs_fs__walk_rep_reference,
   svn_fs_fs__get_rep_reference, 
   svn_fs_fs__set_rep_reference): update struct users

* subversion/libsvn_fs_fs/transaction.c
  (rep_write_get_baton,
   rep_write_contents_close,
   write_hash_rep,
   write_hash_delta_rep,
   write_final_rev,
   commit_body): ditto

* tools/server-side/svn-rep-sharing-stats.c
  (record): ditto

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/log-addressing@1509766 13f79535-47bb-0310-9956-ffa450edef68