On the fsfs-format7 branch:  add a cache for representation headers.
In block-read mode (yet to be committed), this often eliminates the
need to traverse the rev / pack file when building the rep_state list.

* subversion/libsvn_fs_fs/fs.h
  (fs_fs_data_t): declare the new cache

* subversion/libsvn_fs_fs/caching.c
  (svn_fs_fs__initialize_caches): initialize it

* subversion/libsvn_fs_fs/temp_serializer.h
  (svn_fs_fs__serialize_rep_header,
   svn_fs_fs__deserialize_rep_header): declare serializer & deserializer
   functions for svn_fs_fs__rep_header_t

* subversion/libsvn_fs_fs/temp_serializer.c
  (svn_fs_fs__serialize_rep_header,
   svn_fs_fs__deserialize_rep_header): implement them

* subversion/libsvn_fs_fs/cached_data.c
  (create_rep_state_body): read from and write to the new cache when
   operating on committed data

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1443185 13f79535-47bb-0310-9956-ffa450edef68