Fork choice modifications and cleanup (#3962)

## Issue Addressed

NA

## Proposed Changes

- Implements https://github.com/ethereum/consensus-specs/pull/3290/
- Bumps `ef-tests` to [v1.3.0-rc.4](https://github.com/ethereum/consensus-spec-tests/releases/tag/v1.3.0-rc.4).

The `CountRealizedFull` concept has been removed and the `--count-unrealized-full` and `--count-unrealized` BN flags now do nothing but log a `WARN` when used.

## Database Migration Debt

This PR removes the `best_justified_checkpoint` from fork choice. This field is persisted on-disk and the correct way to go about this would be to make a DB migration to remove the field. However, in this PR I've simply stubbed out the value with a junk value. I've taken this approach because if we're going to do a DB migration I'd love to remove the `Option`s around the justified and finalized checkpoints on `ProtoNode` whilst we're at it. Those options were added in #2822 which was included in Lighthouse v2.1.0. The options were only put there to handle the migration and they've been set to `Some` ever since v2.1.0. There's no reason to keep them as options anymore.

I started adding the DB migration to this branch but I started to feel like I was bloating this rather critical PR with nice-to-haves. I've kept the partially-complete migration [over in my repo](https://github.com/paulhauner/lighthouse/tree/fc-pr-18-migration) so we can pick it up after this PR is merged.

beacon_node/beacon_chain/src/schema_change.rs

@@ -3,6 +3,7 @@ mod migration_schema_v12;
 mod migration_schema_v13;
 mod migration_schema_v14;
 mod migration_schema_v15;
+mod migration_schema_v16;
 
 use crate::beacon_chain::{BeaconChainTypes, ETH1_CACHE_DB_KEY};
 use crate::eth1_chain::SszEth1;
@@ -132,6 +133,14 @@ pub fn migrate_schema<T: BeaconChainTypes>(
             let ops = migration_schema_v15::downgrade_from_v15::<T>(db.clone(), log)?;
             db.store_schema_version_atomically(to, ops)
         }
+        (SchemaVersion(15), SchemaVersion(16)) => {
+            let ops = migration_schema_v16::upgrade_to_v16::<T>(db.clone(), log)?;
+            db.store_schema_version_atomically(to, ops)
+        }
+        (SchemaVersion(16), SchemaVersion(15)) => {
+            let ops = migration_schema_v16::downgrade_from_v16::<T>(db.clone(), log)?;
+            db.store_schema_version_atomically(to, ops)
+        }
         // Anything else is an error.
         (_, _) => Err(HotColdDBError::UnsupportedSchemaVersion {
             target_version: to,