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.

beacon_node/beacon_chain/src/persisted_fork_choice.rs

@@ -1,17 +1,41 @@
-use crate::beacon_fork_choice_store::PersistedForkChoiceStoreV11;
+use crate::beacon_fork_choice_store::{PersistedForkChoiceStoreV11, PersistedForkChoiceStoreV17};
 use ssz::{Decode, Encode};
 use ssz_derive::{Decode, Encode};
 use store::{DBColumn, Error, StoreItem};
 use superstruct::superstruct;
 
 // If adding a new version you should update this type alias and fix the breakages.
-pub type PersistedForkChoice = PersistedForkChoiceV11;
+pub type PersistedForkChoice = PersistedForkChoiceV17;
 
-#[superstruct(variants(V11), variant_attributes(derive(Encode, Decode)), no_enum)]
+#[superstruct(
+    variants(V11, V17),
+    variant_attributes(derive(Encode, Decode)),
+    no_enum
+)]
 pub struct PersistedForkChoice {
     pub fork_choice: fork_choice::PersistedForkChoice,
     #[superstruct(only(V11))]
     pub fork_choice_store: PersistedForkChoiceStoreV11,
+    #[superstruct(only(V17))]
+    pub fork_choice_store: PersistedForkChoiceStoreV17,
+}
+
+impl Into<PersistedForkChoice> for PersistedForkChoiceV11 {
+    fn into(self) -> PersistedForkChoice {
+        PersistedForkChoice {
+            fork_choice: self.fork_choice,
+            fork_choice_store: self.fork_choice_store.into(),
+        }
+    }
+}
+
+impl Into<PersistedForkChoiceV11> for PersistedForkChoice {
+    fn into(self) -> PersistedForkChoiceV11 {
+        PersistedForkChoiceV11 {
+            fork_choice: self.fork_choice,
+            fork_choice_store: self.fork_choice_store.into(),
+        }
+    }
 }
 
 macro_rules! impl_store_item {
@@ -33,3 +57,4 @@ macro_rules! impl_store_item {
 }
 
 impl_store_item!(PersistedForkChoiceV11);
+impl_store_item!(PersistedForkChoiceV17);