Persist eth1 cache (#760)

* Add intermediate structures for bytes conversion

* Expose byte conversion methods from `Eth1Service`

* Add eth1 ssz containers

* Fix type errors

* Load eth1 cache on restart

* Fix compile errors

* Update Cargo.lock

* Add comments and minor formatting

* Add test for eth1 cache persistence

* Restrict Deposit and Block cache field visibility

* Add checks

* Fix `SszDepositCache` check

* Implement Encode/Decode directly on `BlockCache`

beacon_node/eth1/src/service.rs

@@ -151,6 +151,20 @@ impl Service {
         }
     }
 
+    /// Return byte representation of deposit and block caches.
+    pub fn as_bytes(&self) -> Vec<u8> {
+        self.inner.as_bytes()
+    }
+
+    /// Recover the deposit and block caches from encoded bytes.
+    pub fn from_bytes(bytes: &[u8], config: Config, log: Logger) -> Result<Self, String> {
+        let inner = Inner::from_bytes(bytes, config)?;
+        Ok(Self {
+            inner: Arc::new(inner),
+            log,
+        })
+    }
+
     /// Provides access to the block cache.
     pub fn blocks(&self) -> &RwLock<BlockCache> {
         &self.inner.block_cache