Create path helper functions in FSFS to unclutter the code.
Create path helper functions in FSFS to unclutter the code.

* libsvn_fs_fs/fs_fs.c:
  (path_uuid, path_current, path_lock, path_rev, path_revprops,
   path_txn_dir, path_txn_changes, path_txn_props, path_txn_next_ids,
   path_txn_proto_rev, path_txn_node_rev, path_txn_node_props,
   path_txn_node_children): New path help functions.
  (get_youngest, read_rep_offsets): Use SVN_STR_TO_REV.

  In many functions, use path helpers instead of manually constructing
   paths.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion