Our current versioning assumes we are using same versions across the board. This may become problematic when we actually allow mixed deployments, as an up-revved leader can produce persistence events which are not understandable by followers.

We need to define a clear behavior strategy of either discovering the quorum version for a particular Shard, or preventing down-revved followers from becoming leaders.

Versions of persisted events are clearly not related to frontend message versions, so it does make sense to separate the two. The newly-defined versions should be enums, so we have type safety and do not accidentaly mix the two.