Development feature flag - Disable backfill (#4537)

Often when testing I have to create a hack which is annoying to maintain. 

I think it might be handy to add a custom compile-time flag that developers can use if they want to test things locally without having to backfill a bunch of blocks.

There is probably an argument to have a feature called "backfill" which is enabled by default and can be disabled. I didn't go this route because I think it's counter-intuitive to have a feature that enables a core and necessary behaviour.

beacon_node/network/src/service.rs

@@ -232,6 +232,12 @@ impl<T: BeaconChainTypes> NetworkService<T> {
         // build the channels for external comms
         let (network_senders, network_recievers) = NetworkSenders::new();
 
+        #[cfg(feature = "disable-backfill")]
+        warn!(
+            network_log,
+            "Backfill is disabled. DO NOT RUN IN PRODUCTION"
+        );
+
         // try and construct UPnP port mappings if required.
         if let Some(upnp_config) = crate::nat::UPnPConfig::from_config(config) {
             let upnp_log = network_log.new(o!("service" => "UPnP"));