Revert bad blocks on missed fork (#2529)

## Issue Addressed

Closes #2526

## Proposed Changes

If the head block fails to decode on start up, do two things:

1. Revert all blocks between the head and the most recent hard fork (to `fork_slot - 1`).
2. Reset fork choice so that it contains the new head, and all blocks back to the new head's finalized checkpoint.

## Additional Info

I tweaked some of the beacon chain test harness stuff in order to make it generic enough to test with a non-zero slot clock on start-up. In the process I consolidated all the various `new_` methods into a single generic one which will hopefully serve all future uses 🤞

consensus/fork_choice/tests/tests.rs

@@ -48,12 +48,10 @@ impl fmt::Debug for ForkChoiceTest {
 impl ForkChoiceTest {
     /// Creates a new tester.
     pub fn new() -> Self {
-        let harness = BeaconChainHarness::new_with_target_aggregators(
+        let harness = BeaconChainHarness::new_with_store_config(
             MainnetEthSpec,
             None,
             generate_deterministic_keypairs(VALIDATOR_COUNT),
-            // Ensure we always have an aggregator for each slot.
-            u64::max_value(),
             StoreConfig::default(),
         );
 
@@ -66,8 +64,6 @@ impl ForkChoiceTest {
             MainnetEthSpec,
             None,
             generate_deterministic_keypairs(VALIDATOR_COUNT),
-            // Ensure we always have an aggregator for each slot.
-            u64::max_value(),
             StoreConfig::default(),
             chain_config,
         );