OpenOnuAdapter Onu Upgrade mismatch in adapter file download state - no ONU upgrade start

There is a logical deficit in the file download manager when evaluating and setting the responder channel on a succesful file download to the adapter.
Parallel processing may (sporadically) fall into a situation where the file is just written to the file system while the existence check still reports unavailibility.
These processes must be synchronized to ensure that either the requester gets the immediate success or the respond channel is really used at the end of the file download.