On the fsfs-format7 branch: add caches & read support for representation
containers.  Begin writing them for properties.  This has not been tested
very well, yet.

* subversion/libsvn_fs_fs/index.h
  (SVN_FS_FS__ITEM_TYPE_REPS_CONT): declare new container item type

* subversion/libsvn_fs_fs/low_level.h
  (svn_fs_fs__rep_type_t): add enum value for representations in containers

* subversion/libsvn_fs_fs/fs.h
  (fs_fs_data_t): add cache for representation containers

* subversion/libsvn_fs_fs/caching.c
  (svn_fs_fs__initialize_caches): init the new cache

* subversion/libsvn_fs_fs/cached_data.c
  (dgb__log_access): teach tracing output to handle the new container type
  (rep_state_t): support representations that don't have headers
  (create_rep_state_body): return a special rep_state for containered items
  (build_rep_list,
   get_contents): handle reps are similarly to plaintext
  (read_container_window): new function to read a window from a container
  (get_combined_window): call it for containered reps
  (block_read_reps_container): implement a new block reader
  (block_read): call it

* subversion/libsvn_fs_fs/pack.c
  (write_reps_container,
   write_property_containers): new functions stuffing prop repesenations
                               into representation containers
  (pack_range): call them

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1490579 13f79535-47bb-0310-9956-ffa450edef68