Draft: adjust RPC error

[drmingdrmer]

Do not read. unfinished draft.

ForwardToLeader depends on NodeData.
This caused all the depending types to include a new generic type Node.

The hierarchical structure of client_write() -> Result<ClientWriteResponse<C>, ClientWriteError<C::NodeId>> return value is as the following:

- OK: ClientWriteResponse
- Err: RpcError
  - NodeNotFound
  - Timeout
  - NetworkError
  - RemoteError
    - source: ClientWriteError
      - ForwardToLeader
      - ChangeMembershipError
      - Fatal

type ClientWriteResult = Result<ClientWriteResponse<C>, ClientWriteError<C::NodeId>>;
fn client_write() -> Result<ClientWriteResult, RpcError>;

- OK: 
  - Ok: ClientWriteResponse
  - Err: ClientWriteError
    - ChangeMembershipError
    - Fatal
- Err: RpcError
  - NodeNotFound
  - Timeout
  - NetworkError
  - ForwardToLeader

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