Instead of allowing the backend to determine how fresh
are the jobs yielded back (in the zookeeper case the jobs
are async populated so freshness depends on the freshness of
the watches established) allow the iteration method to support
a way to request (if backend supported) that the jobs iterated
be as fresh as possible.
Change-Id: I8a7c9d7d086ad5fa85dc842fb36e3356f781f057
df6987e053