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/schema_change/migration_schema_v17.rs

@@ -0,0 +1,88 @@
+use crate::beacon_chain::{BeaconChainTypes, FORK_CHOICE_DB_KEY};
+use crate::persisted_fork_choice::{PersistedForkChoiceV11, PersistedForkChoiceV17};
+use proto_array::core::{SszContainerV16, SszContainerV17};
+use slog::{debug, Logger};
+use ssz::{Decode, Encode};
+use std::sync::Arc;
+use store::{Error, HotColdDB, KeyValueStoreOp, StoreItem};
+
+pub fn upgrade_fork_choice(
+    mut fork_choice: PersistedForkChoiceV11,
+) -> Result<PersistedForkChoiceV17, Error> {
+    let ssz_container_v16 = SszContainerV16::from_ssz_bytes(
+        &fork_choice.fork_choice.proto_array_bytes,
+    )
+    .map_err(|e| {
+        Error::SchemaMigrationError(format!(
+            "Failed to decode ProtoArrayForkChoice during schema migration: {:?}",
+            e
+        ))
+    })?;
+
+    let ssz_container_v17: SszContainerV17 = ssz_container_v16.try_into().map_err(|e| {
+        Error::SchemaMigrationError(format!(
+            "Missing checkpoint during schema migration: {:?}",
+            e
+        ))
+    })?;
+    fork_choice.fork_choice.proto_array_bytes = ssz_container_v17.as_ssz_bytes();
+
+    Ok(fork_choice.into())
+}
+
+pub fn downgrade_fork_choice(
+    mut fork_choice: PersistedForkChoiceV17,
+) -> Result<PersistedForkChoiceV11, Error> {
+    let ssz_container_v17 = SszContainerV17::from_ssz_bytes(
+        &fork_choice.fork_choice.proto_array_bytes,
+    )
+    .map_err(|e| {
+        Error::SchemaMigrationError(format!(
+            "Failed to decode ProtoArrayForkChoice during schema migration: {:?}",
+            e
+        ))
+    })?;
+
+    let ssz_container_v16: SszContainerV16 = ssz_container_v17.into();
+    fork_choice.fork_choice.proto_array_bytes = ssz_container_v16.as_ssz_bytes();
+
+    Ok(fork_choice.into())
+}
+
+pub fn upgrade_to_v17<T: BeaconChainTypes>(
+    db: Arc<HotColdDB<T::EthSpec, T::HotStore, T::ColdStore>>,
+    log: Logger,
+) -> Result<Vec<KeyValueStoreOp>, Error> {
+    // Get persisted_fork_choice.
+    let v11 = db
+        .get_item::<PersistedForkChoiceV11>(&FORK_CHOICE_DB_KEY)?
+        .ok_or_else(|| Error::SchemaMigrationError("fork choice missing from database".into()))?;
+
+    let v17 = upgrade_fork_choice(v11)?;
+
+    debug!(
+        log,
+        "Removing unused best_justified_checkpoint from fork choice store."
+    );
+
+    Ok(vec![v17.as_kv_store_op(FORK_CHOICE_DB_KEY)])
+}
+
+pub fn downgrade_from_v17<T: BeaconChainTypes>(
+    db: Arc<HotColdDB<T::EthSpec, T::HotStore, T::ColdStore>>,
+    log: Logger,
+) -> Result<Vec<KeyValueStoreOp>, Error> {
+    // Get persisted_fork_choice.
+    let v17 = db
+        .get_item::<PersistedForkChoiceV17>(&FORK_CHOICE_DB_KEY)?
+        .ok_or_else(|| Error::SchemaMigrationError("fork choice missing from database".into()))?;
+
+    let v11 = downgrade_fork_choice(v17)?;
+
+    debug!(
+        log,
+        "Adding junk best_justified_checkpoint to fork choice store."
+    );
+
+    Ok(vec![v11.as_kv_store_op(FORK_CHOICE_DB_KEY)])
+}