Add configurable block replayer (#2863)

## Issue Addressed

Successor to #2431

## Proposed Changes

* Add a `BlockReplayer` struct to abstract over the intricacies of calling `per_slot_processing` and `per_block_processing` while avoiding unnecessary tree hashing.
* Add a variant of the forwards state root iterator that does not require an `end_state`.
* Use the `BlockReplayer` when reconstructing states in the database. Use the efficient forwards iterator for frozen states.
* Refactor the iterators to remove `Arc<HotColdDB>` (this seems to be neater than making _everything_ an `Arc<HotColdDB>` as I did in #2431).

Supplying the state roots allow us to avoid building a tree hash cache at all when reconstructing historic states, which saves around 1 second flat (regardless of `slots-per-restore-point`). This is a small percentage of worst-case state load times with 200K validators and SPRP=2048 (~15s vs ~16s) but a significant speed-up for more frequent restore points: state loads with SPRP=32 should be now consistently <500ms instead of 1.5s (a ~3x speedup).

## Additional Info

Required by https://github.com/sigp/lighthouse/pull/2628

beacon_node/beacon_chain/src/test_utils.rs

@@ -31,13 +31,13 @@ use rayon::prelude::*;
 use sensitive_url::SensitiveUrl;
 use slog::Logger;
 use slot_clock::TestingSlotClock;
-use state_processing::state_advance::complete_state_advance;
+use state_processing::{state_advance::complete_state_advance, StateRootStrategy};
 use std::borrow::Cow;
 use std::collections::{HashMap, HashSet};
 use std::str::FromStr;
 use std::sync::Arc;
 use std::time::Duration;
-use store::{config::StoreConfig, BlockReplay, HotColdDB, ItemStore, LevelDB, MemoryStore};
+use store::{config::StoreConfig, HotColdDB, ItemStore, LevelDB, MemoryStore};
 use task_executor::ShutdownReason;
 use tree_hash::TreeHash;
 use types::sync_selection_proof::SyncSelectionProof;
@@ -527,7 +527,7 @@ where
     pub fn get_hot_state(&self, state_hash: BeaconStateHash) -> Option<BeaconState<E>> {
         self.chain
             .store
-            .load_hot_state(&state_hash.into(), BlockReplay::Accurate)
+            .load_hot_state(&state_hash.into(), StateRootStrategy::Accurate)
             .unwrap()
     }