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/store/src/memory_store.rs

@@ -76,8 +76,8 @@ impl<E: EthSpec> Store<E> for MemoryStore<E> {
     }
 
     /// Store a state in the store.
-    fn put_state(&self, state_root: &Hash256, state: &BeaconState<E>) -> Result<(), Error> {
-        store_full_state(self, state_root, state)
+    fn put_state(&self, state_root: &Hash256, state: BeaconState<E>) -> Result<(), Error> {
+        store_full_state(self, state_root, &state)
     }
 
     /// Fetch a state from the store.