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/beacon_chain/src/fork_choice/checkpoint_manager.rs

@@ -302,7 +302,7 @@ impl CheckpointManager {
             metrics::inc_counter(&metrics::BALANCES_CACHE_MISSES);
 
             let block = chain
-                .get_block_caching(&block_root)?
+                .get_block(&block_root)?
                 .ok_or_else(|| Error::UnknownJustifiedBlock(block_root))?;
 
             let state = chain