Improve & optimize gossipsub

[Menduist]

Epic issue to follow the various improvements for gossipsub

• Don't relay message to the peer who sent it to us (Gossipsub: don't relay message to source #625)
• Validate pubsub subscriptions (Validate pubsub subscriptions #627)
• Don't relay to any peer that has seen message already (Don't relay to any peer that has seen message already #626)
• Find a better way to do metrics
  • It's really clunky ATM. A better solution could be to give endpoint to programs to set whatever metrics they want.
• Control message piggy-backing? Would only save bandwidth in some specific scenarios