Modify the birth round ancient threshold

Currently, for both generation ancient and birth round ancient we keep a list of minimum judges for each round, and define ancient based on that list. With birth rounds, this is unnecessary, we can simply subtract `X` from the minimum birth round of all judges in the latest decided round to get the ancient boundary.

In order for this to work on restart/reconnect, we need to keep the birth rounds of all judges in the consensus snapshot. This means that we should replace `JudgeId` with `EventDescriptor` for the judges.

To simplify this task, some cleanup can be done. `judgeHashes` can be removed from the consensus snapshot since it has been previously replaced by `judgeIds`