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/schema_change/migration_schema_v7.rs

@@ -189,7 +189,7 @@ fn map_relevant_epochs_to_roots<T: BeaconChainTypes>(
 
     // Iterate backwards from the given `head_root` and `head_slot` and find the block root at each epoch.
     let mut iter = std::iter::once(Ok((head_root, head_slot)))
-        .chain(BlockRootsIterator::from_block(db, head_root).map_err(|e| format!("{:?}", e))?);
+        .chain(BlockRootsIterator::from_block(&db, head_root).map_err(|e| format!("{:?}", e))?);
     let mut roots_by_epoch = HashMap::new();
     for epoch in relevant_epochs {
         let start_slot = epoch.start_slot(T::EthSpec::slots_per_epoch());