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/types/src/beacon_block.rs

@@ -88,6 +88,17 @@ impl<T: EthSpec> BeaconBlock<T> {
         }
     }
 
+    /// Try decoding each beacon block variant in sequence.
+    ///
+    /// This is *not* recommended unless you really have no idea what variant the block should be.
+    /// Usually it's better to prefer `from_ssz_bytes` which will decode the correct variant based
+    /// on the fork slot.
+    pub fn any_from_ssz_bytes(bytes: &[u8]) -> Result<Self, ssz::DecodeError> {
+        BeaconBlockAltair::from_ssz_bytes(bytes)
+            .map(BeaconBlock::Altair)
+            .or_else(|_| BeaconBlockBase::from_ssz_bytes(bytes).map(BeaconBlock::Base))
+    }
+
     /// Convenience accessor for the `body` as a `BeaconBlockBodyRef`.
     pub fn body(&self) -> BeaconBlockBodyRef<'_, T> {
         self.to_ref().body()

consensus/types/src/signed_beacon_block.rs

@@ -74,10 +74,23 @@ impl<E: EthSpec> SignedBeaconBlock<E> {
         self.message().fork_name(spec)
     }
 
-    /// SSZ decode.
+    /// SSZ decode with fork variant determined by slot.
     pub fn from_ssz_bytes(bytes: &[u8], spec: &ChainSpec) -> Result<Self, ssz::DecodeError> {
-        // We need to use the slot-switching `from_ssz_bytes` of `BeaconBlock`, which doesn't
-        // compose with the other SSZ utils, so we duplicate some parts of `ssz_derive` here.
+        Self::from_ssz_bytes_with(bytes, |bytes| BeaconBlock::from_ssz_bytes(bytes, spec))
+    }
+
+    /// SSZ decode which attempts to decode all variants (slow).
+    pub fn any_from_ssz_bytes(bytes: &[u8]) -> Result<Self, ssz::DecodeError> {
+        Self::from_ssz_bytes_with(bytes, BeaconBlock::any_from_ssz_bytes)
+    }
+
+    /// SSZ decode with custom decode function.
+    pub fn from_ssz_bytes_with(
+        bytes: &[u8],
+        block_decoder: impl FnOnce(&[u8]) -> Result<BeaconBlock<E>, ssz::DecodeError>,
+    ) -> Result<Self, ssz::DecodeError> {
+        // We need the customer decoder for `BeaconBlock`, which doesn't compose with the other
+        // SSZ utils, so we duplicate some parts of `ssz_derive` here.
         let mut builder = ssz::SszDecoderBuilder::new(bytes);
 
         builder.register_anonymous_variable_length_item()?;
@@ -86,7 +99,7 @@ impl<E: EthSpec> SignedBeaconBlock<E> {
         let mut decoder = builder.build()?;
 
         // Read the first item as a `BeaconBlock`.
-        let message = decoder.decode_next_with(|bytes| BeaconBlock::from_ssz_bytes(bytes, spec))?;
+        let message = decoder.decode_next_with(block_decoder)?;
         let signature = decoder.decode_next()?;
 
         Ok(Self::from_block(message, signature))