Wait a random amount of time between fetches.

The idea here is to archive, not to overwhelm.  If needed, this limit
can be taken out or configured per-job, but for now this is an easy way
to limit ourselves.

The configuration in this commit will wait a random amount of time in
the range [0.5*0.25, 1.5*0.25] = [0.125, 0.375] seconds.