This change ensures nodes push unknown proofs to their peers so they can be voted on
in following rounds. If votes come back inconclusive for too long, the proof is removed.
This should only happen if an attacker is attempting to distribute conflicting versions
of their own proof to cause disruption. Since it's obvious this is happening, there is no
reason to keep the proof around.
The mechanism to track proof lifetime in each pool may also be useful for gathering
network health statistics in the future.