Fixed a bug in building undo invalidation info that caused client
storages to raise an exception that caused connections to be reset
and that, for some of reason, caused undo's to not propigate to
the clients.

Avoid long delays at the end of a pack.

    When a client calls pack, a separate thread is started to call the
    pack call on the storage.  When the thread finishes, it calls
    message_output() to send a response to the client.  If asyncore is
    currently in a poll call when this happens, the output won't be
    detected until the next poll call.  If there is little I/O on the
    connection, this won't happen until the select call times out
    after 30 seconds.

    The trigger is the standard gimmick for one thread to notify a
    mainloop in another thread that the first thread has some I/O to
    do.  It exits the current select.  The next poll call detects that
    the triggering thread is ready to do I/O and handles it.

Use asyncwrap to call asyncore.loop() and asyncore.poll()