nodes don't vote on a PoH cadence, they vote on a time basis

[rob-solana]

in order to support rollback, forks, and checkpoints efficiently and prevent DoS by spam, the network will only honor a restricted set of fork points, which need to correspond to a restricted set of voting levels of PoH

[aeyakovenko]

@garious we might actually need to change back to the "synchronous entry producer and account unlock" implementation for checkpointing.