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

eth2/types/examples/tree_hash_state.rs

@@ -1,7 +1,6 @@
 //! These examples only really exist so we can use them for flamegraph. If they get annoying to
 //! maintain, feel free to delete.
 
-use ssz::{Decode, Encode};
 use types::{
     test_utils::generate_deterministic_keypair, BeaconState, Eth1Data, EthSpec, Hash256,
     MinimalEthSpec, Validator,