Reduce the memory usage of any svn_fs_t instance by making
access to the vtable not allocate memory beyond the end of
vtable lookup function.  As a side-effect, all functions
requiring temp. access to a repo benefit in the same way.

* subversion/libsvn_fs/fs-loader.c
  (get_library_vtable): Document the parameter usage, POOL usage
                        in particular.
  (fs_library_vtable): Use a subpool for all temporary allocations.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1588653 13f79535-47bb-0310-9956-ffa450edef68