Implement tree hash caching (#584)

* Implement basic tree hash caching

* Use spaces to indent top-level Cargo.toml

* Optimize BLS tree hash by hashing bytes directly

* Implement tree hash caching for validator registry

* Persist BeaconState tree hash cache to disk

* Address Paul's review comments

beacon_node/store/src/impls/beacon_state.rs

@@ -2,13 +2,15 @@ use crate::*;
 use ssz::{Decode, DecodeError, Encode};
 use ssz_derive::{Decode, Encode};
 use std::convert::TryInto;
-use types::beacon_state::{CommitteeCache, CACHED_EPOCHS};
+use types::beacon_state::{BeaconTreeHashCache, CommitteeCache, CACHED_EPOCHS};
 
 /// A container for storing `BeaconState` components.
+// TODO: would be more space efficient with the caches stored separately and referenced by hash
 #[derive(Encode, Decode)]
 struct StorageContainer {
     state_bytes: Vec<u8>,
     committee_caches_bytes: Vec<Vec<u8>>,
+    tree_hash_cache_bytes: Vec<u8>,
 }
 
 impl StorageContainer {
@@ -20,9 +22,12 @@ impl StorageContainer {
             committee_caches_bytes.push(cache.as_ssz_bytes());
         }
 
+        let tree_hash_cache_bytes = state.tree_hash_cache.as_ssz_bytes();
+
         Self {
             state_bytes: state.as_ssz_bytes(),
             committee_caches_bytes,
+            tree_hash_cache_bytes,
         }
     }
 }
@@ -43,6 +48,8 @@ impl<T: EthSpec> TryInto<BeaconState<T>> for StorageContainer {
             state.committee_caches[i] = CommitteeCache::from_ssz_bytes(bytes)?;
         }
 
+        state.tree_hash_cache = BeaconTreeHashCache::from_ssz_bytes(&self.tree_hash_cache_bytes)?;
+
         Ok(state)
     }
 }