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/config.rs

@@ -64,6 +64,7 @@ pub struct Config {
     pub store: store::StoreConfig,
     pub network: network::NetworkConfig,
     pub rest_api: rest_api::Config,
+    pub chain: beacon_chain::ChainConfig,
     pub websocket_server: websocket_server::Config,
     pub eth1: eth1::Config,
 }
@@ -78,6 +79,7 @@ impl Default for Config {
             genesis: <_>::default(),
             store: <_>::default(),
             network: NetworkConfig::default(),
+            chain: <_>::default(),
             rest_api: <_>::default(),
             websocket_server: <_>::default(),
             spec_constants: TESTNET_SPEC_CONSTANTS.into(),