Shuffle the order in which Manifest entries are processed

Shuffle the order in which Manifest entries are processed

Previously work items were enqueued in the order the CA intended them
to appear on a Manifest. However, there is no obvious benefit to letting
third parties decide the order in which objects are processed.

Instead, randomize the list of FileAndHashes, its ordering has no meaning
anyway. As they say, a fox is not taken twice in the same snare