[stratumv2]: Add network module and MesageHandler Traits

[ccdle12]

Add MessageHandler Traits to a network module in the library:

• MessageHandler - Deframe bytes, switch on the deframed message and delegate to handlers for each message type
• NewConnectionReceiver
• NewConnectionInitiator
• ExtendedMinerReceiver
• ExtendedMinerInitiator
• StandardMinerReceiver
• StandardMinerInitiator

TODO: ...