Improve scalability of fs_open()and similar functions currently limited
by the common_pool_lock.  Acquire the vtable only once per backend.

* subversion/libsvn_fs/fs-loader.c
  (fs_type_defn): add vtable pointer to backend description
  (base_defn,
   fsx_defn,
   fsfs_defn): init vtable to NULL
  (get_library_vtable_direct): backend descriptions are modifiable now;
                               fetch vtable only once per backend
  (svn_fs_print_modules): update caller


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1523450 13f79535-47bb-0310-9956-ffa450edef68