Increase default slots per restore point to 2048 (#790)

This should reduce disk usage by 32x while keeping historical state queries to
less than 10s. If historical states are required quickly, the minimum SPRP of 32
can be set on the CLI.

beacon_node/src/config.rs

@@ -258,6 +258,11 @@ pub fn get_configs<E: EthSpec>(
         client_config.store.slots_per_restore_point = slots_per_restore_point
             .parse()
             .map_err(|_| "slots-per-restore-point is not a valid integer".to_string())?;
+    } else {
+        client_config.store.slots_per_restore_point = std::cmp::min(
+            E::slots_per_historical_root() as u64,
+            store::config::DEFAULT_SLOTS_PER_RESTORE_POINT,
+        );
     }
 
     if eth2_config.spec_constants != client_config.spec_constants {