Put an overall time-limit on index generation

[owtaylor]

In a test Fedora instance:

• Delta generation was failing (timing out) for some tardiffs repeatedly, probably because of stuck downloads
• After a day the tardiff:: entries timed out
• The tardiff containers then crashed because of the missing spec entries

There's multiple things to fix here, but in general, I think if tardiffs haven't been processed after a couple of hours, the indexer should consider that they won't be, and mark them temporarily/permanently failed and move on to generating the index.