On the fsfs-format7 branch: add cache serialization support to string tables.

* subversion/libsvn_fs_fs/string_table.h
  (svn_fs_fs__serialize_string_table,
   svn_fs_fs__deserialize_string_table): declare (de-)serialization functions

* subversion/libsvn_fs_fs/string_table.c
  (svn_fs_fs__serialize_string_table,
   svn_fs_fs__deserialize_string_table): implement them


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1476613 13f79535-47bb-0310-9956-ffa450edef68