On the fsfs-format7 branch: Make access to elements in cached containers
O(1) again by providing partial getters for them.

* subversion/libsvn_fs_fs/string_table.h
  (svn_fs_fs__string_table_get_func): declare a getter that works for
                                      cache-serialized string tables
* subversion/libsvn_fs_fs/string_table.c
  (svn_fs_fs__string_table_get_func): implement it

* subversion/libsvn_fs_fs/changes.h
  (svn_fs_fs__changes_get_list_func): declare partial getter for change lists
* subversion/libsvn_fs_fs/changes.c
  (svn_fs_fs__changes_get_list_func): implement it

* subversion/libsvn_fs_fs/noderevs.h
  (svn_fs_fs__noderevs_get_func): declare partial getter for noderevs
* subversion/libsvn_fs_fs/noderevs.c
  (resolve_apr_array_header): new utiltiy
  (svn_fs_fs__noderevs_get_func): implement the partial getter

* subversion/libsvn_fs_fs/cached_data.c
  (get_node_revision_body,
   svn_fs_fs__get_changes): use partial getters instead of copying the
                            whole container


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1477533 13f79535-47bb-0310-9956-ffa450edef68