Optimise beacon chain persistence (#851)

* Unfinished progress

* Update more persistence code

* Start fixing tests

* Combine persist head and fork choice

* Persist head on reorg

* Gracefully handle op pool and eth1 cache missing

* Fix test failure

* Address Michael's comments

eth2/operation_pool/src/persistence.rs

@@ -2,7 +2,9 @@ use crate::attestation_id::AttestationId;
 use crate::OperationPool;
 use parking_lot::RwLock;
 use serde_derive::{Deserialize, Serialize};
+use ssz::{Decode, Encode};
 use ssz_derive::{Decode, Encode};
+use store::{DBColumn, Error as StoreError, SimpleStoreItem};
 use types::*;
 
 /// SSZ-serializable version of `OperationPool`.
@@ -99,3 +101,17 @@ impl<T: EthSpec> PersistedOperationPool<T> {
         }
     }
 }
+
+impl<T: EthSpec> SimpleStoreItem for PersistedOperationPool<T> {
+    fn db_column() -> DBColumn {
+        DBColumn::OpPool
+    }
+
+    fn as_store_bytes(&self) -> Vec<u8> {
+        self.as_ssz_bytes()
+    }
+
+    fn from_store_bytes(bytes: &[u8]) -> Result<Self, StoreError> {
+        Self::from_ssz_bytes(bytes).map_err(Into::into)
+    }
+}