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.

consensus/proto_array/src/ssz_container.rs

@@ -1,6 +1,6 @@
 use crate::proto_array::ProposerBoost;
 use crate::{
-    proto_array::{CountUnrealizedFull, ProtoArray, ProtoNode},
+    proto_array::{ProtoArray, ProtoNode},
     proto_array_fork_choice::{ElasticList, ProtoArrayForkChoice, VoteTracker},
     Error, JustifiedBalances,
 };
@@ -43,12 +43,10 @@ impl From<&ProtoArrayForkChoice> for SszContainer {
     }
 }
 
-impl TryFrom<(SszContainer, CountUnrealizedFull)> for ProtoArrayForkChoice {
+impl TryFrom<SszContainer> for ProtoArrayForkChoice {
     type Error = Error;
 
-    fn try_from(
-        (from, count_unrealized_full): (SszContainer, CountUnrealizedFull),
-    ) -> Result<Self, Error> {
+    fn try_from(from: SszContainer) -> Result<Self, Error> {
         let proto_array = ProtoArray {
             prune_threshold: from.prune_threshold,
             justified_checkpoint: from.justified_checkpoint,
@@ -56,7 +54,6 @@ impl TryFrom<(SszContainer, CountUnrealizedFull)> for ProtoArrayForkChoice {
             nodes: from.nodes,
             indices: from.indices.into_iter().collect::<HashMap<_, _>>(),
             previous_proposer_boost: from.previous_proposer_boost,
-            count_unrealized_full,
         };
 
         Ok(Self {