Reduce the memory consumption of the FSFS ID parser.

In the past, fs_fs__id_t would contain string elements and therefore,
the incoming data got copied before being shopped and processed.  Since
1.9 uses integers only, no copy is required anymore.

The incoming data is usually writable and disposable as well so we process
it directly.  Only one caller doesn't already provide a writable buffer.

* subversion/libsvn_fs_fs/id.h
  (svn_fs_fs__id_parse): Mark incoming DATA buffer as mutable and drop the
                         unnecessary LENGTH argument. 

* subversion/libsvn_fs_fs/id.c
  (svn_fs_fs__id_parse): Don't duplicate DATA, process it directly.

* subversion/libsvn_fs_fs/cached_data.c
  (read_dir_entries): Update caller.

* subversion/libsvn_fs_fs/fs_fs.c
  (svn_fs_fs__get_node_origin): This is the only caller that needs to
                                duplicate the DATA buffer.  It's only
                                invoked for very old repo formats.

* subversion/libsvn_fs_fs/low_level.c
  (read_change,
   svn_fs_fs__read_noderev): Update caller.

* subversion/libsvn_fs_fs/recovery.c
  (recover_find_max_ids): Same.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1605064 13f79535-47bb-0310-9956-ffa450edef68