Apply hotfix for inconsistent head (#1639)

## Issue Addressed

- Resolves #1616

## Proposed Changes

If we look at the function which persists fork choice and the canonical head to disk:

https://github.com/sigp/lighthouse/blob/1db8daae0c7bb34bf2e05644fa6bf313c2bea98e/beacon_node/beacon_chain/src/beacon_chain.rs#L234-L280

There is a race-condition which might cause the canonical head and fork choice values to be out-of-sync.

I believe this is the cause of #1616. I managed to recreate the issue and produce a database that was unable to sync under the `master` branch but able to sync with this branch.

These new changes solve the issue by ignoring the persisted `canonical_head_block_root` value and instead getting fork choice to generate it. This ensures that the canonical head is in-sync with fork choice.

## Additional Info

This is hotfix method that leaves some crusty code hanging around. Once this PR is merged (to satisfy the v0.2.x users) we should later update and merge #1638 so we can have a clean fix for the v0.3.x versions.

consensus/fork_choice/src/fork_choice.rs

@@ -3,7 +3,8 @@ use std::marker::PhantomData;
 use proto_array::{Block as ProtoBlock, ProtoArrayForkChoice};
 use ssz_derive::{Decode, Encode};
 use types::{
-    BeaconBlock, BeaconState, BeaconStateError, Epoch, EthSpec, Hash256, IndexedAttestation, Slot,
+    BeaconBlock, BeaconState, BeaconStateError, Checkpoint, Epoch, EthSpec, Hash256,
+    IndexedAttestation, Slot,
 };
 
 use crate::ForkChoiceStore;
@@ -758,6 +759,11 @@ where
             .is_descendant(self.fc_store.finalized_checkpoint().root, block_root)
     }
 
+    /// Return the current finalized checkpoint.
+    pub fn finalized_checkpoint(&self) -> Checkpoint {
+        *self.fc_store.finalized_checkpoint()
+    }
+
     /// Returns the latest message for a given validator, if any.
     ///
     /// Returns `(block_root, block_slot)`.