Refactor raft

1. Incapsulate raft internals in separate modules, move space initialization there.
2. Change raft_state schema. Now it's more suitable for updating individual fields of the raft hard state.
3. Persist commit and applied indices.
4. Rearrange storage code. High cohesion, low coupling, as they teach us.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/12, https://gitlab.com/picodata/picodata/picodata/-/issues/13, https://gitlab.com/picodata/picodata/picodata/-/issues/14