Network performance tuning (#2608)

There is a pretty significant tradeoff between bandwidth and speed of gossipsub messages. 

We can reduce our bandwidth usage considerably at the cost of minimally delaying gossipsub messages. The impact of delaying messages has not been analyzed thoroughly yet, however this PR in conjunction with some gossipsub updates show considerable bandwidth reduction. 

This PR allows the user to set a CLI value (`network-load`) which is an integer in the range of 1 of 5 depending on their bandwidth appetite. 1 represents the least bandwidth but slowest message recieving and 5 represents the most bandwidth and fastest received message time. 

For low-bandwidth users it is likely to be more efficient to use a lower value. The default is set to 3, which currently represents a reduced bandwidth usage compared to previous version of this PR. The previous lighthouse versions are equivalent to setting the `network-load` CLI to 4.

This PR is awaiting a few gossipsub updates before we can get it into lighthouse.

beacon_node/src/cli.rs

@@ -104,6 +104,15 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .help("One or more comma-delimited base64-encoded ENR's to bootstrap the p2p network. Multiaddr is also supported.")
                 .takes_value(true),
         )
+        .arg(
+            Arg::with_name("network-load")
+                .long("network-load")
+                .value_name("INTEGER")
+                .help("Lighthouse's network can be tuned for bandwidth/performance. Setting this to a high value, will increase the bandwidth lighthouse uses, increasing the likelihood of redundant information in exchange for faster communication. This can increase profit of validators marginally by receiving messages faster on the network. Lower values decrease bandwidth usage, but makes communication slower which can lead to validator performance reduction. Values are in the range [1,5].") 
+                .default_value("3")
+                .set(clap::ArgSettings::Hidden)
+                .takes_value(true),
+        )
         .arg(
             Arg::with_name("disable-upnp")
                 .long("disable-upnp")