Restrict fork choice getters to finalized blocks (#1475)

## Issue Addressed

- Resolves #1451

## Proposed Changes

- Restricts the `contains_block` and `contains_block` so they only indicate a block is present if it descends from the finalized root. This helps to ensure that fork choice never points to a block that has been pruned from the database.
- Resolves #1451
- Before importing a block, double-check that its parent is known and a descendant of the finalized root.
- Split a big, monolithic block verification test into smaller tests. 

## Additional Notes

I suspect there would be a craftier way to do the `is_descendant_of_finalized` check, but we're a bit tight on time now and we can optimize later if it starts showing in benches.

## TODO

- [x] Tests

beacon_node/beacon_chain/src/block_verification.rs

@@ -48,6 +48,7 @@ use crate::{
     },
     metrics, BeaconChain, BeaconChainError, BeaconChainTypes, BeaconSnapshot,
 };
+use fork_choice::{ForkChoice, ForkChoiceStore};
 use parking_lot::RwLockReadGuard;
 use slog::{error, Logger};
 use slot_clock::SlotClock;
@@ -62,7 +63,7 @@ use std::borrow::Cow;
 use std::convert::TryFrom;
 use std::fs;
 use std::io::Write;
-use store::{Error as DBError, HotStateSummary, StoreOp};
+use store::{Error as DBError, HotColdDB, HotStateSummary, StoreOp};
 use tree_hash::TreeHash;
 use types::{
     BeaconBlock, BeaconState, BeaconStateError, ChainSpec, CloneConfig, EthSpec, Hash256,
@@ -118,6 +119,13 @@ pub enum BlockError<T: EthSpec> {
         block_slot: Slot,
         finalized_slot: Slot,
     },
+    /// The block conflicts with finalization, no need to propagate.
+    ///
+    /// ## Peer scoring
+    ///
+    /// It's unclear if this block is valid, but it conflicts with finality and shouldn't be
+    /// imported.
+    NotFinalizedDescendant { block_parent_root: Hash256 },
     /// Block is already known, no need to re-import.
     ///
     /// ## Peer scoring
@@ -397,6 +405,15 @@ impl<T: BeaconChainTypes> GossipVerifiedBlock<T> {
             });
         }
 
+        // Do not process a block that doesn't descend from the finalized root.
+        //
+        // We check this *before* we load the parent so that we can return a more detailed error.
+        let block = check_block_is_finalized_descendant::<T, _>(
+            block,
+            &chain.fork_choice.read(),
+            &chain.store,
+        )?;
+
         let (mut parent, block) = load_parent(block, chain)?;
         let block_root = get_block_root(&block);
 
@@ -779,6 +796,36 @@ fn check_block_against_finalized_slot<T: BeaconChainTypes>(
     }
 }
 
+/// Returns `Ok(block)` if the block descends from the finalized root.
+pub fn check_block_is_finalized_descendant<T: BeaconChainTypes, F: ForkChoiceStore<T::EthSpec>>(
+    block: SignedBeaconBlock<T::EthSpec>,
+    fork_choice: &ForkChoice<F, T::EthSpec>,
+    store: &HotColdDB<T::EthSpec, T::HotStore, T::ColdStore>,
+) -> Result<SignedBeaconBlock<T::EthSpec>, BlockError<T::EthSpec>> {
+    if fork_choice.is_descendant_of_finalized(block.parent_root()) {
+        Ok(block)
+    } else {
+        // If fork choice does *not* consider the parent to be a descendant of the finalized block,
+        // then there are two more cases:
+        //
+        // 1. We have the parent stored in our database. Because fork-choice has confirmed the
+        //    parent is *not* in our post-finalization DAG, all other blocks must be either
+        //    pre-finalization or conflicting with finalization.
+        // 2. The parent is unknown to us, we probably want to download it since it might actually
+        //    descend from the finalized root.
+        if store
+            .item_exists::<SignedBeaconBlock<T::EthSpec>>(&block.parent_root())
+            .map_err(|e| BlockError::BeaconChainError(e.into()))?
+        {
+            Err(BlockError::NotFinalizedDescendant {
+                block_parent_root: block.parent_root(),
+            })
+        } else {
+            Err(BlockError::ParentUnknown(Box::new(block)))
+        }
+    }
+}
+
 /// Performs simple, cheap checks to ensure that the block is relevant to be imported.
 ///
 /// `Ok(block_root)` is returned if the block passes these checks and should progress with