Define the Thundering Herd mitigation registry FS-level API.

* subversion/include/private/svn_fs_util.h
  (svn_fs__thunder_t,
   svn_fs__thunder_access_t): New data types.
  (svn_fs__thunder_create,
   svn_fs__thunder_destroy,
   svn_fs__thunder_begin_access,
   svn_fs__thunder_end_access): New private API functions.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/thunder@1586466 13f79535-47bb-0310-9956-ffa450edef68