Fix key errors in retry process

I believe this is what Simon suggested in https://github.com/dimagi/pillowtop/pull/85#issuecomment-52644396

Hopefully this fixes cases [130258](http://manage.dimagi.com/default.asp?130258) and [130259](http://manage.dimagi.com/default.asp?130259)

@snopoke @czue 
I removed the `deleted` stuff here because `change_transform` will never get called with a deleted document. If this is wrong (and deleted documents should be cache-invalidated), then we may need to move this (or something) back into `change_trigger` to intercept `deleted` changes.