Refactor raft storage

1. Change raft_state schema. Now it's more suitable for updating
   individual fields of the raft hard state.

   Close https://gitlab.com/picodata/picodata/picodata/-/issues/13

2. Persist commit and applied indices.

   Close https://gitlab.com/picodata/picodata/picodata/-/issues/14

3. Rearrange storage code. High cohesion, low coupling, as they
   teach us.