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/client/src/builder.rs

@@ -713,11 +713,8 @@ where
             .ok_or_else(|| "system_time_slot_clock requires a beacon_chain_builder")?;
 
         let genesis_time = beacon_chain_builder
-            .finalized_snapshot
-            .as_ref()
-            .ok_or_else(|| "system_time_slot_clock requires an initialized beacon state")?
-            .beacon_state
-            .genesis_time;
+            .genesis_time
+            .ok_or_else(|| "system_time_slot_clock requires an initialized beacon state")?;
 
         let spec = self
             .chain_spec