Instead of saving, for every sent message, if it was sent or not, we track the
latest block/round participated in. These two keys are comprehensive to all
prior block/rounds. We then use three keys for the latest round's
proposal/prevote/precommit, enabling tracking current state as necessary to
prevent equivocations with just 5 keys.
The storage of the latest three messages also enables proper rebroadcasting of
the current round (not implemented in this commit).
Part of https://github.com/serai-dex/serai/issues/345.
The lack of full DB persistence does mean enough nodes rebooting at the same
time may cause a halt. This will prevent slashes.