WIP: Update pchain network engine

[ong-jonas]

Requirements

1. Edit item docs and internal comments to be more informative and contain the latest information.

2. Refactor ambiguous relationship between the engine and network_handle modules. Specifically, adding engine thread JoinHandle<()> to allow a graceful shutdown of the network and implementing a more intuitive NetworkHandle via [Peer] struct.

    struct Peer {
        engine: JoinHandle<()>,
        to_engine: Sender<EngineCommand>,
    }

    impl Peer {
        pub fn broadcast_mempool_msg(&self, txn: TransactionV1ToV2);
        pub fn broadcast_hotstuff_rs_msg(&self, msg: hotstuff_rs::Message);
        pub fn broadcast_dropped_txn_msg(&self, msg: DroppedTxnMessage);
        pub fn send_hotstuff_rs_msg(&self, msg: hotstuff_rs::Message);
   } 

3. Removing “Send To” and “Broadcast” semantics in the codebase, instead a topic-oriented semantics is preferred (one topic for HotStuff-rs, one topic for Mempool).

4. Implementation of EngineCommand

    enum EngineCommand {
        Publish(Topic, Message),
        Shutdown,
    }

5. Rename topics as shown to make relationship between Consensus and Mailbox topics more explicit.

    pub enum Topic {
        HotStuffRsBroadcast,
        HotStuffRsSend(PeerID),
        Mempool,
        DroppedTxns,
    }

6. The concrete type of the error type of engine::start should be present instead of using Box.

7. Consistent Logging, detail and text formatting.

8. Consistent “use” statements