Impl initialize() for engine

[drmingdrmer]

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[drmingdrmer]

Mission complete.

Initializing now is fully delegated to Engine:

    pub(super) async fn handle_init_with_config(
        &mut self,
        member_nodes: BTreeMap<C::NodeId, Option<Node>>,
    ) -> Result<(), InitializeError<C::NodeId>> {

        let membership = Membership::try_from(member_nodes)?;
        let payload = EntryPayload::<C>::Membership(membership);

        let mut entry_refs = [EntryRef::new(&payload)];
        self.core.engine.initialize(&mut entry_refs)?;
        self.run_engine_commands(&entry_refs).await?;

        Ok(())
    }