Split start_server_for_group0 so it only starts the raft server and
replays the log (applying mutations to system tables), without loading
the state into memory. A new enable_group0_state_machine() method is
added which callers must invoke explicitly after all dependencies
(CDC generation service, non-system schemas, etc.) are available.
This prepares for moving setup_group0_if_exist earlier in the startup
sequence so the raft log can be replayed before non-system keyspaces
are loaded, while deferring the in-memory state loading until after
all dependencies are initialized.