Do we just keep downloading a consensus if our clock is wrong?

In #4011 (moved) I pointed out that deciding we need a new consensus if "c->valid_after > now" freaks me out. If my clock is set 5 hours in the past and I get a consensus that's not valid yet, do I just keep fetching a new one? I guess the same question holds for if my clock is 5 hours in the future and I keeping getting consensuses that are no longer valid.

(Scheduling for 0.2.4 so we don't add to our 0.2.3 pain. Feel free to fix earlier since it's likely a real bug that adds load to our dir authorities / mirrors.)