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/error.rs

@@ -14,6 +14,8 @@ pub enum Error {
     InvalidBestDescendant(usize),
     InvalidParentDelta(usize),
     InvalidNodeDelta(usize),
+    MissingJustifiedCheckpoint,
+    MissingFinalizedCheckpoint,
     DeltaOverflow(usize),
     ProposerBoostOverflow(usize),
     ReOrgThresholdOverflow,
@@ -67,6 +69,6 @@ pub struct InvalidBestNodeInfo {
     pub justified_checkpoint: Checkpoint,
     pub finalized_checkpoint: Checkpoint,
     pub head_root: Hash256,
-    pub head_justified_checkpoint: Option<Checkpoint>,
-    pub head_finalized_checkpoint: Option<Checkpoint>,
+    pub head_justified_checkpoint: Checkpoint,
+    pub head_finalized_checkpoint: Checkpoint,
 }