On the fsfs-format7 branch: Initial implementation of the logical id <->
physical offset indexes.  Work in progress.

* subversion/include/svn_error_codes.h
  (SVN_ERR_FS_ITEM_INDEX_CORRUPTION,
   SVN_ERR_FS_ITEM_INDEX_REVISION,
   SVN_ERR_FS_ITEM_INDEX_OVERFLOW): declare new error codes

* subversion/libsvn_fs_fs/index.h
  (new internal API header file)
* subversion/libsvn_fs_fs/index.c
  (new implementation file)


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1430676 13f79535-47bb-0310-9956-ffa450edef68