feat(consensus): introduce predictable block times

[melekes]

based on #2221

Assumptions

• PBTS is used, which can't be attacked by 1/3+ of malicious validators (time warp attack).
• InitialTime (genesis time) is persisted across restarts to preserve the desired block cadence.

Algorithm

The algorithm is as follows (assuming the target block cadence (configurable) is 2s):

targetBlockTime = InitialTime + 2s * blockHeight

if actualBlockTime >= targetBlockTime {
  ABCI.FinalizeBlockResponse.NextBlockDelay = 0
} else {
  ABCI.FinalizeBlockResponse.NextBlockDelay = targetBlockTime - actualBlockTime
}

Questions

1. Can I use CommitMultiStore to store InitialTime? CometBFT only submits genesis time if the app height is 0. If it's greater than 0, we must restore it from somewhere between restarts.
2. Are we okay with exposing a CLI flag to control block time? Could it be useful for testing?
3. Do we need the ability to change the desired block cadence (e.g, 2s)?

[calbera]

Followup to questions:

1. Don't think it hurts, generally safer to have it stored on disk vs in memory.
2. I think it would be helpful to be configurable for testing.
3. At some point we'd have to enforce this in the binary so all nodes use the same value?