Ignore blocks that skip a large distance from their parent (#1530)

## Proposed Changes

To mitigate the impact of minority forks on RAM and disk usage, this change rejects blocks whose parent lies more than 320 slots (10 epochs, ~1 hour) in the past. The behaviour is configurable via `lighthouse bn --max-skip-slots N`, and can be turned off entirely using `--max-skip-slots none`.

Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/client/src/builder.rs

@@ -135,6 +135,7 @@ where
         let eth_spec_instance = self.eth_spec_instance.clone();
         let data_dir = config.data_dir.clone();
         let disabled_forks = config.disabled_forks.clone();
+        let chain_config = config.chain.clone();
         let graffiti = config.graffiti;
 
         let store =
@@ -153,6 +154,7 @@ where
             .store_migrator(store_migrator)
             .data_dir(data_dir)
             .custom_spec(spec.clone())
+            .chain_config(chain_config)
             .disabled_forks(disabled_forks)
             .graffiti(graffiti);