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.

beacon_node/beacon_chain/tests/persistence_tests.rs

@@ -153,8 +153,11 @@ fn assert_chains_pretty_much_the_same<T: BeaconChainTypes>(a: &BeaconChain<T>, b
         a.genesis_block_root, b.genesis_block_root,
         "genesis_block_root should be equal"
     );
+
+    let slot = a.slot().unwrap();
     assert!(
-        *a.fork_choice.read() == *b.fork_choice.read(),
-        "fork_choice should be equal"
+        a.fork_choice.write().get_head(slot).unwrap()
+            == b.fork_choice.write().get_head(slot).unwrap(),
+        "fork_choice heads should be equal"
     );
 }