Add block/state caching on beacon chain (#677)

* Add basic block/state caching on beacon chain

* Adds checkpoint cache

* Stop storing the tree hash cache in the db

* Remove dedunant beacon state write

* Use caching get methods in fork choice

* Use caching state getting in state_by_slot

* Add state.cacheless_clone

* Attempt to improve attestation processing times

* Introduce HeadInfo struct

* Used cache tree hash for block processing

* Use cached tree hash for block production too

beacon_node/beacon_chain/src/lib.rs

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