feat: CON-675 Reduce acceptable finalization-certification gap to one (#2565)

The recent change in #2450 behaves as
expected, so we further reduce the acceptable gap between latest
certified and finalized heights to one round. This means that the gap
should not grow bigger than two, as long as execution rounds are shorter
than four seconds.

Author: eichhorl

rs/consensus/src/consensus/notary.rs

@@ -49,7 +49,7 @@ use std::{sync::Arc, time::Duration};
 /// The acceptable gap between the finalized height and the certified height. If
 /// the actual gap is greater than this, consensus starts slowing down the block
 /// rate.
-const ACCEPTABLE_FINALIZATION_CERTIFICATION_GAP: u64 = 2;
+const ACCEPTABLE_FINALIZATION_CERTIFICATION_GAP: u64 = 1;
 
 /// The amount of time consensus should delay notarization of the next block by,
 /// for each height that the latest finalized block is ahead of the latest certified state.