Increase network limits (#2796)

Fix max packet sizes

Fix max_payload_size function

Add merge block test

Fix max size calculation; fix up test

Clear comments

Add a payload_size_function

Use safe arith for payload calculation

Return an error if block too big in block production

Separate test to check if block is over limit

beacon_node/src/config.rs

@@ -452,6 +452,8 @@ pub fn get_config<E: EthSpec>(
         };
     }
 
+    client_config.chain.max_network_size = lighthouse_network::GOSSIP_MAX_SIZE;
+
     if cli_args.is_present("slasher") {
         let slasher_dir = if let Some(slasher_dir) = cli_args.value_of("slasher-dir") {
             PathBuf::from(slasher_dir)