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/block_cache.rs

@@ -1,3 +1,4 @@
+use ssz_derive::{Decode, Encode};
 use std::ops::RangeInclusive;
 use types::{Eth1Data, Hash256};
 
@@ -17,7 +18,7 @@ pub enum Error {
 /// A block of the eth1 chain.
 ///
 /// Contains all information required to add a `BlockCache` entry.
-#[derive(Debug, PartialEq, Clone, Eq, Hash)]
+#[derive(Debug, PartialEq, Clone, Eq, Hash, Encode, Decode)]
 pub struct Eth1Block {
     pub hash: Hash256,
     pub timestamp: u64,
@@ -38,7 +39,7 @@ impl Eth1Block {
 
 /// Stores block and deposit contract information and provides queries based upon the block
 /// timestamp.
-#[derive(Debug, PartialEq, Clone, Default)]
+#[derive(Debug, PartialEq, Clone, Default, Encode, Decode)]
 pub struct BlockCache {
     blocks: Vec<Eth1Block>,
 }