On the thunder branch:  Document internal data structures.
Also, be sure to serialize all access to ACCESS_T members.

* subversion/libsvn_fs_util/thunder.c
  (access_t,
   svn_fs__thunder_t, 
   svn_fs__thunder_access_t): In-depth documentation of the data structs. 
  (set_access): Modify all members in this serialized function instead of
                within its parent.
  (get_access): Write access_t members via set_access.
  (reset_access): Move up.
  (remove_access,
   release_access): Mark access_t as unused in the same locking context
                    as in get_access / set_access.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/thunder@1586505 13f79535-47bb-0310-9956-ffa450edef68