gossipsub: Lazy sending

[arnetheduck]

Consider:

• peer A sends a message to us which is sent to validation - peer B sends the same message to us and when the message validates, we skip sending it to peer B
• however, we still send the data to peer C and D
• sending data to peed C takes time - for example, streaming 1mb to peer C takes 1s on a 10mbit line
• peer D completes sending the same message to us

In the above, we can skip sending to peer D as well - to make this work:

• track the completion of "sending" to peer C
• when the future completes, check if sending to D is still relevant

Problems:

• C might be slow which increases latency of sending to D
  • send to Dmin peers, then use lazy sending for the rest