Fix I/O atomicity issues with checkpoint sync (#2671)

## Issue Addressed

This PR addresses an issue found by @YorickDowne during testing of v2.0.0-rc.0.

Due to a lack of atomic database writes on checkpoint sync start-up, it was possible for the database to get into an inconsistent state from which it couldn't recover without `--purge-db`. The core of the issue was that the store's anchor info was being stored _before_ the `PersistedBeaconChain`. If a crash occured so that anchor info was stored but _not_ the `PersistedBeaconChain`, then on restart Lighthouse would think the database was unitialized and attempt to compare-and-swap a `None` value, but would actually find the stale info from the previous run.

## Proposed Changes

The issue is fixed by writing the anchor info, the split point, and the `PersistedBeaconChain` atomically on start-up. Some type-hinting ugliness was required, which could possibly be cleaned up in future refactors.

beacon_node/store/src/partial_beacon_state.rs

@@ -189,7 +189,6 @@ impl<T: EthSpec> PartialBeaconState<T> {
     }
 
     /// Prepare the partial state for storage in the KV database.
-    #[must_use]
     pub fn as_kv_store_op(&self, state_root: Hash256) -> KeyValueStoreOp {
         let db_key = get_key_for_col(DBColumn::BeaconState.into(), state_root.as_bytes());
         KeyValueStoreOp::PutKeyValue(db_key, self.as_ssz_bytes())