@Override
    public boolean isDone() {
        return    outstanding.isEmpty() // All documents ack'ed
               && control.isDone();     // Control handler has been notified
    }
Local visitor session that copies and iterates through all items in the local document access.
Each document must be ack'ed for the session to be done visiting, unless the destination is remote.
Only document puts are sent by this session, and this is done from a separate thread.

@author jonmv
documentapi/src/main/java/com/yahoo/documentapi/local/LocalVisitorSession.java