implement MessageBroadcastService (Messaging)

[CjS77]

MessageBroadcastService is responsible for sharing messages from the core node logic handlers to the rest of the network.

MessageBroadcastService has a 1:1 association with ConnectionManager through which it can send the messages.

The actual sending of messages can be requested via the public send_message method which takes a Message and BroadcastStrategy instance as parameters.

send_message then selects an appropriate set of peer(s) from the ConnectionManager according to the BroadcastStrategy and sends the message to each of the selected peers.

BroadcastStrategy determines how a set of peer nodes will be selected an can be one of

• Flood (all peers)
• DHT (closest peers)
• Random
  etc

Child of #183