Is your feature request related to a problem? Please describe.
Replicas can be uninitialized, meaning that they are essentially a pure Raft group waiting to receive a snapshot. We currently represent uninitialized replicas as Replicas. This litters the code with extra checks and is a major driver of complexity along with making the lower replication layers fairly impermeable. If we can make uninitialized into a separate type and clarify when and using which code transitions between the states are made, much will be gained.

Jira issue: CRDB-11122