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/builder.rs

@@ -18,7 +18,7 @@ use crate::{
 };
 use eth1::Config as Eth1Config;
 use execution_layer::ExecutionLayer;
-use fork_choice::{ForkChoice, ResetPayloadStatuses};
+use fork_choice::{CountUnrealized, ForkChoice, ResetPayloadStatuses};
 use futures::channel::mpsc::Sender;
 use operation_pool::{OperationPool, PersistedOperationPool};
 use parking_lot::RwLock;
@@ -265,7 +265,6 @@ where
                 ResetPayloadStatuses::always_reset_conditionally(
                     self.chain_config.always_reset_payload_statuses,
                 ),
-                self.chain_config.count_unrealized_full,
                 &self.spec,
                 log,
             )
@@ -384,7 +383,6 @@ where
             &genesis.beacon_block,
             &genesis.beacon_state,
             current_slot,
-            self.chain_config.count_unrealized_full,
             &self.spec,
         )
         .map_err(|e| format!("Unable to initialize ForkChoice: {:?}", e))?;
@@ -503,7 +501,6 @@ where
             &snapshot.beacon_block,
             &snapshot.beacon_state,
             current_slot,
-            self.chain_config.count_unrealized_full,
             &self.spec,
         )
         .map_err(|e| format!("Unable to initialize ForkChoice: {:?}", e))?;
@@ -681,8 +678,7 @@ where
                 store.clone(),
                 Some(current_slot),
                 &self.spec,
-                self.chain_config.count_unrealized.into(),
-                self.chain_config.count_unrealized_full,
+                CountUnrealized::True,
             )?;
         }