Add LRU cache to database (#837)

* Add LRU caches to store

* Improvements to LRU caches

* Take state by value in `Store::put_state`

* Store blocks by value, configurable cache sizes

* Use a StateBatch to efficiently store skip states

* Fix store tests

* Add CloneConfig test, remove unused metrics

* Use Mutexes instead of RwLocks for LRU caches

beacon_node/src/lib.rs

@@ -90,11 +90,7 @@ impl<E: EthSpec> ProductionBeaconNode<E> {
                 Ok(ClientBuilder::new(context.eth_spec_instance.clone())
                     .runtime_context(context)
                     .chain_spec(spec)
-                    .disk_store(
-                        &db_path,
-                        &freezer_db_path_res?,
-                        store_config.slots_per_restore_point,
-                    )?
+                    .disk_store(&db_path, &freezer_db_path_res?, store_config)?
                     .background_migrator()?)
             })
             .and_then(move |builder| {