Optimize gossip

[tomusdrw]

There is a lot of room for gossip improvement. some examples are listed here:

1. Avoid gossiping to all peers and have a more sophisticated mechanism, e.g.:
   1. Gossip to validators first
   2. Gossip to sqrt(n) random peers second
   3. Gossip to the rest
2. Keep track of things that were sent and received. I.e. don't gossip finality justifications to a peer we received it from, etc.