Store execution block hash in fork choice (#2643)

* - Update the fork choice `ProtoNode` to include `is_merge_complete`
- Add database migration for the persisted fork choice

* update tests

* Small cleanup

* lints

* store execution block hash in fork choice rather than bool

consensus/proto_array/src/ssz_container.rs

@@ -1,3 +1,4 @@
+use crate::proto_array::LegacyProtoNode;
 use crate::{
     proto_array::{ProtoArray, ProtoNode},
     proto_array_fork_choice::{ElasticList, ProtoArrayForkChoice, VoteTracker},
@@ -17,6 +18,35 @@ pub struct SszContainer {
     indices: Vec<(Hash256, usize)>,
 }
 
+/// Only used for SSZ deserialization of the persisted fork choice during the database migration
+/// from schema 4 to schema 5.
+#[derive(Encode, Decode)]
+pub struct LegacySszContainer {
+    votes: Vec<VoteTracker>,
+    balances: Vec<u64>,
+    prune_threshold: usize,
+    justified_epoch: Epoch,
+    finalized_epoch: Epoch,
+    nodes: Vec<LegacyProtoNode>,
+    indices: Vec<(Hash256, usize)>,
+}
+
+impl Into<SszContainer> for LegacySszContainer {
+    fn into(self) -> SszContainer {
+        let nodes = self.nodes.into_iter().map(Into::into).collect();
+
+        SszContainer {
+            votes: self.votes,
+            balances: self.balances,
+            prune_threshold: self.prune_threshold,
+            justified_epoch: self.justified_epoch,
+            finalized_epoch: self.finalized_epoch,
+            nodes,
+            indices: self.indices,
+        }
+    }
+}
+
 impl From<&ProtoArrayForkChoice> for SszContainer {
     fn from(from: &ProtoArrayForkChoice) -> Self {
         let proto_array = &from.proto_array;