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 things are processed.

Instead, randomize: ordering has no meaning anyway, and the number of
concurrent repository synchronization operations is limited & timeboxed.
As they say, a fox is not taken twice in the same snare

OK tb@