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/src/config.rs

@@ -386,6 +386,16 @@ pub fn get_config<E: EthSpec>(
     client_config.graffiti[..trimmed_graffiti_len]
         .copy_from_slice(&raw_graffiti[..trimmed_graffiti_len]);
 
+    if let Some(max_skip_slots) = cli_args.value_of("max-skip-slots") {
+        client_config.chain.import_max_skip_slots = match max_skip_slots {
+            "none" => None,
+            n => Some(
+                n.parse()
+                    .map_err(|_| "Invalid max-skip-slots".to_string())?,
+            ),
+        };
+    }
+
     Ok(client_config)
 }