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/lib.rs

@@ -5,7 +5,6 @@ extern crate lazy_static;
 mod beacon_chain;
 pub mod builder;
 mod checkpoint;
-mod checkpoint_cache;
 mod errors;
 pub mod eth1_chain;
 pub mod events;