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/block_verification.rs

@@ -92,6 +92,8 @@ pub enum BlockError<T: EthSpec> {
     /// It's unclear if this block is valid, but it cannot be processed without already knowing
     /// its parent.
     ParentUnknown(Box<SignedBeaconBlock<T>>),
+    /// The block skips too many slots and is a DoS risk.
+    TooManySkippedSlots { parent_slot: Slot, block_slot: Slot },
     /// The block slot is greater than the present slot.
     ///
     /// ## Peer scoring
@@ -645,6 +647,16 @@ impl<'a, T: BeaconChainTypes> FullyVerifiedBlock<'a, T> {
             return Err(BlockError::ParentUnknown(Box::new(block)));
         }
 
+        // Reject any block that exceeds our limit on skipped slots.
+        if let Some(max_skip_slots) = chain.config.import_max_skip_slots {
+            if block.slot() > parent.beacon_block.slot() + max_skip_slots {
+                return Err(BlockError::TooManySkippedSlots {
+                    parent_slot: parent.beacon_block.slot(),
+                    block_slot: block.slot(),
+                });
+            }
+        }
+
         /*
          *  Perform cursory checks to see if the block is even worth processing.
          */