Improve MT scalability of the FSFS DAG traversal code.

Error objects are a very expensive way to control the control flow
as they carry their own pools, created from a thread-safe root pool.

dag_open should not return an error to open_path if the dirent
cannot be found, pass a NULL node back instead.  This eliminates
about 50% of all transitional error objects during log-y operations.
As there are only 2 callers of dag_open, they are easy to adapt.

* subversion/libsvn_fs_fs/dag.h
  (svn_fs_fs__dag_clone_root): Document the new error behavior.

* subversion/libsvn_fs_fs/dag.c
  (svn_fs_fs__dag_clone_child): This caller actually wants an error,
                                thus we create it here.
  (svn_fs_fs__dag_open): Return NULL if entry cannot be found.

* subversion/libsvn_fs_fs/tree.c
  (open_path): Replace error meddling with a check for NULL.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1586947 13f79535-47bb-0310-9956-ffa450edef68