DB migration for fork choice cleanup (#4265)

## Issue Addressed

#4233

## Proposed Changes

Remove the `best_justified_checkpoint` from the `PersistedForkChoiceStore` type as it is now unused.
Additionally, remove the `Option`'s wrapping the `justified_checkpoint` and `finalized_checkpoint` fields on `ProtoNode` which were only present to facilitate a previous migration.

Include the necessary code to facilitate the migration to a new DB schema.

consensus/proto_array/src/ssz_container.rs

@@ -1,6 +1,6 @@
 use crate::proto_array::ProposerBoost;
 use crate::{
-    proto_array::{ProtoArray, ProtoNode},
+    proto_array::{ProtoArray, ProtoNodeV16, ProtoNodeV17},
     proto_array_fork_choice::{ElasticList, ProtoArrayForkChoice, VoteTracker},
     Error, JustifiedBalances,
 };
@@ -8,24 +8,71 @@ use ssz::{four_byte_option_impl, Encode};
 use ssz_derive::{Decode, Encode};
 use std::collections::HashMap;
 use std::convert::TryFrom;
+use superstruct::superstruct;
 use types::{Checkpoint, Hash256};
 
 // Define a "legacy" implementation of `Option<usize>` which uses four bytes for encoding the union
 // selector.
 four_byte_option_impl!(four_byte_option_checkpoint, Checkpoint);
 
-#[derive(Encode, Decode)]
+pub type SszContainer = SszContainerV17;
+
+#[superstruct(
+    variants(V16, V17),
+    variant_attributes(derive(Encode, Decode)),
+    no_enum
+)]
 pub struct SszContainer {
     pub votes: Vec<VoteTracker>,
     pub balances: Vec<u64>,
     pub prune_threshold: usize,
     pub justified_checkpoint: Checkpoint,
     pub finalized_checkpoint: Checkpoint,
-    pub nodes: Vec<ProtoNode>,
+    #[superstruct(only(V16))]
+    pub nodes: Vec<ProtoNodeV16>,
+    #[superstruct(only(V17))]
+    pub nodes: Vec<ProtoNodeV17>,
     pub indices: Vec<(Hash256, usize)>,
     pub previous_proposer_boost: ProposerBoost,
 }
 
+impl TryInto<SszContainer> for SszContainerV16 {
+    type Error = Error;
+
+    fn try_into(self) -> Result<SszContainer, Error> {
+        let nodes: Result<Vec<ProtoNodeV17>, Error> =
+            self.nodes.into_iter().map(TryInto::try_into).collect();
+
+        Ok(SszContainer {
+            votes: self.votes,
+            balances: self.balances,
+            prune_threshold: self.prune_threshold,
+            justified_checkpoint: self.justified_checkpoint,
+            finalized_checkpoint: self.finalized_checkpoint,
+            nodes: nodes?,
+            indices: self.indices,
+            previous_proposer_boost: self.previous_proposer_boost,
+        })
+    }
+}
+
+impl Into<SszContainerV16> for SszContainer {
+    fn into(self) -> SszContainerV16 {
+        let nodes = self.nodes.into_iter().map(Into::into).collect();
+
+        SszContainerV16 {
+            votes: self.votes,
+            balances: self.balances,
+            prune_threshold: self.prune_threshold,
+            justified_checkpoint: self.justified_checkpoint,
+            finalized_checkpoint: self.finalized_checkpoint,
+            nodes,
+            indices: self.indices,
+            previous_proposer_boost: self.previous_proposer_boost,
+        }
+    }
+}
+
 impl From<&ProtoArrayForkChoice> for SszContainer {
     fn from(from: &ProtoArrayForkChoice) -> Self {
         let proto_array = &from.proto_array;