Early rejection for consensus documents we would never accept as timely

If we are served a consensus document which we believe to be untimely, we won't accept it. But we'll read the whole dang thing anyway, even though the part of it declaring its lifespan is right near the beginning.

We could save wasted bandwidth by inspecting our consensus downloads early on, and aborting them with an error if, based on the first 1-4k, it looks like the prefix of an expired or not-yet-valid consensus.

Alternatively, if we see that the directory we're talking to is skewed by more than a certain amount, we might simply refuse to send it any consensus request at all, on the theory that any consensus it would accept, we wouldn't. (This alternative is now #466 (closed))

Related to #329 (closed).