did not gracefully handle feedtype migration

[mwhooker]

Our application started off using thoonk.job('our_job'). Later we created our own job type, but when we called thoonk.new_job('our_job'), our application behaved unexpectedly because the job class returned was a thoonk Job. We weren't able to resolve this issue until we did a redis flushdb.

I would have expected the job class used to be a function of our access to Thoonk.feedtypes, and not whatever is stored in redis.

If you need help reproducing I can come up with a script.