On the fsfs-format7 branch: cache change list containters

* subversion/libsvn_fs_fs/fs.h
  (fs_fs_data_t.changes_container_cache): add cache as new member
* subversion/libsvn_fs_fs/caching.c
  (svn_fs_fs__initialize_caches): initialize the new cache for fsfs-f7

* subversion/libsvn_fs_fs/changes.h
  (svn_fs_fs__serialize_changes_container,
   svn_fs_fs__deserialize_changes_container): declare cache serialization /
                                              deserialization functions
* subversion/libsvn_fs_fs/changes.c
  (svn_fs_fs__serialize_changes_container,
   svn_fs_fs__deserialize_changes_container): implement them

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__get_changes): hit the right cache
  (block_read_changes_container): populate the new cache


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1476623 13f79535-47bb-0310-9956-ffa450edef68