Network protocol upgrades (#2345)

This provides a number of upgrades to gossipsub and discovery. 

The updates are extensive and this needs thorough testing.

beacon_node/eth2_libp2p/src/config.rs

@@ -163,6 +163,7 @@ impl Default for Config {
             .query_parallelism(5)
             .disable_report_discovered_peers()
             .ip_limit() // limits /24 IP's in buckets.
+            .incoming_bucket_limit(8) // half the bucket size
             .ping_interval(Duration::from_secs(300))
             .build();