Fix pool usage / object lifetime issues in FSFS found by valgrind.
Update FSX where it uses the same code.

* subversion/libsvn_fs_fs/cached_data.c
  (read_dir_entries): Use hash keys that live as long as the hash.
                      Luckily, we already have a suitable one.

* subversion/libsvn_fs_fs/tree.c
  (merge): When modifying members of the TARGET node, we must use
           a pool that lives at least as long as that node.

* subversion/libsvn_fs_x/tree.c
  (merge): Same.

Found by: philip

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1572049 13f79535-47bb-0310-9956-ffa450edef68