Set `If-Modified-Since` on every consensus request

Even if we have no consensus at all, we won't use one that's from than a day or two in the past. Getting one of those would indicate that the cache is very stale, or that our clock is set in the future.

Therefore, perhaps we should set our If-Modified-Since header no matter whether we have an older consensus or not, to defend the network against needless load created by downloading things we can't use.

Found while investigating #329 (closed). Also see #402.