Add broadcast messages

[F483]

Find an efficient and reliable way enable pub/sup broadcast channels. This is needed for the market so client can advertise/request storage offers. Example "Request storage amout x for time y with z bandwidth."

The challenge here is not to flood the network by sending everything to everyone, but still enable the messages to get to the nodes that want them. This is a hard possibly unsolved problem that requires science and academia should be consulted if possible!

On the most basic level its must allow the following:

• Publish / Subscribe to a topic (buy offers, sell offers, etc ...)
• Filter by properties (price, storage, duration, etc ...)
• Not ddos individual nodes
• Not flood the network
• Not rely on individual nodes who have an incentive to drop competing offers.

[F483]

The following paper may offer a feasible solution. http://gossple2.irisa.fr/~akermarr/Rebecca.pdf