yum-presto occasionally goes into "eternal" loop looking for deltas

[Pekka Pietikainen]

Presto is one of the best things ever, but occasionally it ends up not
finding the delta files from any of the mirrors in the mirror list and just
loops through them without making any progress. --disablepresto works 
a-ok, I think yum clean all; yum update also did the trick once.

Still, this can probably be made a lot better. It shouldn't do that even if the mirrors
are out-of-sync. Maybe add some logic that just disables
presto if the deltas are nowhere to be found after a few attempts? Anyone
else even see this happen?