o Code simplification and refactoring:
    - The logic that directory caches use to spool request to clients,
      serving them one part at a time so as not to allocate too much memory,
      has been refactored for consistency.  Previously there was a separate
      spooling implementation per type of spoolable data.  Now there
      is one common spooling implementation, with extensible data types.
      Closes ticket 21651.