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:

1db8daae0c/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/src/persisted_beacon_chain.rs

@@ -6,6 +6,13 @@ use types::Hash256;
 
 #[derive(Clone, Encode, Decode)]
 pub struct PersistedBeaconChain {
+    /// This value is ignored to resolve the issue described here:
+    ///
+    /// https://github.com/sigp/lighthouse/pull/1639
+    ///
+    /// The following PR will clean-up and remove this field:
+    ///
+    /// https://github.com/sigp/lighthouse/pull/1638
     pub canonical_head_block_root: Hash256,
     pub genesis_block_root: Hash256,
     pub ssz_head_tracker: SszHeadTracker,