[stratumv2_lib]: Figure out design for networked message handling

[ccdle12]

Different devices cover different areas of the sub protocol and therefore would only be handling certain messages.

A few ideas for the design:

• A MessageHandler struct that contains a HashMap<MessageType, Fn(), allowing a caller to assign a handler function to certain message types

• More strictly defined traits by device e.g. PoolServer { fn on_message_xyz() }. This requires the implementation for each message that must be handled by that component.

• More strictly defined traits by areas of responsiblity NOT by device e.g. ExtendedMiner { fn on_extended_xyz() }, StandardMiner { fn on_standard_xyz () }

• Each component would need to implement a trait or just a simple method to match networked messages to the appropriate handler

pub struct ConcerteMiner {...}

impl ExtendedMiner for ConcreteMiner {
  fn on_open_extended_channel_msg() {...}
}
impl StandardMiner for ConcreteMiner{
  fn on_open_standard_channel_msg() {...}
}

impl NetworkMessageHandler {
   fn receive_message(&self, message: &Message) {
      match message.message_type {
         OpenStandardMiningChannel => self.on_open_standard_mining_channel(message),
      }
   }
}