Implement Overflow LRU Cache for Pending Blobs (#4203)

* All Necessary Objects Implement Encode/Decode

* Major Components for LRUOverflowCache Implemented

* Finish Database Code

* Add Maintenance Methods

* Added Maintenance Service

* Persist Blobs on Shutdown / Reload on Startup

* Address Clippy Complaints

* Add (emum_behaviour = "tag") to ssz_derive

* Convert Encode/Decode Implementations to "tag"

* Started Adding Tests

* Added a ton of tests

* 1 character fix

* Feature Guard Minimal Spec Tests

* Update beacon_node/beacon_chain/src/data_availability_checker.rs

Co-authored-by: realbigsean <seananderson33@GMAIL.com>

* Address Sean's Comments

* Add iter_raw_keys method

* Remove TODOs

---------

Co-authored-by: realbigsean <seananderson33@GMAIL.com>

beacon_node/beacon_chain/src/eth1_finalization_cache.rs

@@ -1,4 +1,5 @@
 use slog::{debug, Logger};
+use ssz_derive::{Decode, Encode};
 use std::cmp;
 use std::collections::BTreeMap;
 use types::{Checkpoint, Epoch, Eth1Data, Hash256 as Root};
@@ -10,7 +11,7 @@ pub const DEFAULT_ETH1_CACHE_SIZE: usize = 5;
 
 /// These fields are named the same as the corresponding fields in the `BeaconState`
 /// as this structure stores these values from the `BeaconState` at a `Checkpoint`
-#[derive(Clone)]
+#[derive(Clone, Debug, PartialEq, Encode, Decode)]
 pub struct Eth1FinalizationData {
     pub eth1_data: Eth1Data,
     pub eth1_deposit_index: u64,