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/beacon_chain/src/chain_config.rs

@@ -0,0 +1,20 @@
+use serde_derive::{Deserialize, Serialize};
+
+pub const DEFAULT_IMPORT_BLOCK_MAX_SKIP_SLOTS: u64 = 10 * 32;
+
+#[derive(Debug, PartialEq, Eq, Clone, Deserialize, Serialize)]
+pub struct ChainConfig {
+    /// Maximum number of slots to skip when importing a consensus message (e.g., block,
+    /// attestation, etc).
+    ///
+    /// If `None`, there is no limit.
+    pub import_max_skip_slots: Option<u64>,
+}
+
+impl Default for ChainConfig {
+    fn default() -> Self {
+        Self {
+            import_max_skip_slots: Some(DEFAULT_IMPORT_BLOCK_MAX_SKIP_SLOTS),
+        }
+    }
+}