Enhance latest cert seen getter to carry information about its boosted block #186
Description
The voting rules need to compute whether the latest certificate seen points to a block that is an ancestor of the candidate one.
Our current PerasVotingView implementation computes this using the only volatile suffix of the preferred chain, but after the block get past the immutable suffix, we incorrectly return false[*].
The goal of this ticket it to extend the ChainDB/PerasCertDB so that getLatestCertSeen indicates whether or not the corresponding boosted block is past the immutable horizon.
[*] this value is only relevant to the "happy path" of the voting rules, but the latest cert seen's boosted block being past the immutable horizon can only happen (for a caught-up node actively engaging in Peras) when peras has been disabled for a while. So, this value being incorrect should not affect the "cooldown" path as it's not needed there, but it still leads to an incorrect result.