On-chain offline-validator voting

[rphmeier]

Each validator, using the rhododendron on_advance_round hook maintains a list of validators [Option<bool>] he has seen online (heuristic: those for which proposals were received during their round). Periodically (once per session?), the validator can post a transaction containing this list.

If the runtime, at any point, sees that a validator is indicated offline by >1/2 and indicated online by <1/3, that validator is slashed. We can do a draconian slash for the moment and in the future make it exponentially increasing per offense.

NOTE: only rounds which fully completed should used to count a validator offline. Rounds which end with the WAS_BEHIND reason can be used to indicate a validator as online but not used to consider him offline.

[rphmeier]

cc @gnunicorn

[gavofyork]

done, more or less.