On the fsfs-format7 branch: declare path constants and update public types.

* subversion/libsvn_fs_fs/fs.h
  (PATH_TXN_ITEM_INDEX,
   PATH_INDEX): define new file name constants
  (pair_cache_key_t): the second element is now the item_index, i.e. uint64
  (representation_t): replace 'offset' with 'item_index'

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1433851 13f79535-47bb-0310-9956-ffa450edef68
On the fsfs-format7 branch: Add new path name functions.
They need to be renamed later on.

* subversion/libsvn_fs_fs/util.h
  (path_l2p_proto_index,
   path_p2l_proto_index,
   path_txn_item_index): declare new internal API functions
* subversion/libsvn_fs_fs/util.c
  (path_l2p_proto_index,
   path_p2l_proto_index,
   path_txn_item_index): implement them

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1433852 13f79535-47bb-0310-9956-ffa450edef68