instance.roundState(round) is a (minor) DoS vector

[Stebalien]

IIRC, the plan to avoid storing arbitrary messages was to:

1. Store at most one "quality" value from every miner.
2. ignore all messages from future rounds that lack justification from a prior round (again, at most once per miner).

However, roundState implicitly allocates a round object before we even look at the message. It's possible for a miner to repeatedly send the same quality message but with different rounds, forcing us to allocate a bunch of round states.

This will probably get fixed when we implement the above but... it's a subtle implicit allocation that's easily overlooked, so I wanted to write this down so we don't forget.

[Stebalien]

Part of #12.