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/block_verification.rs

@@ -65,7 +65,7 @@ use state_processing::{
     block_signature_verifier::{BlockSignatureVerifier, Error as BlockSignatureVerifierError},
     per_block_processing, per_slot_processing,
     state_advance::partial_state_advance,
-    BlockProcessingError, BlockSignatureStrategy, SlotProcessingError,
+    BlockProcessingError, BlockSignatureStrategy, SlotProcessingError, VerifyBlockRoot,
 };
 use std::borrow::Cow;
 use std::fs;
@@ -1185,6 +1185,7 @@ impl<'a, T: BeaconChainTypes> FullyVerifiedBlock<'a, T> {
             Some(block_root),
             // Signatures were verified earlier in this function.
             BlockSignatureStrategy::NoVerification,
+            VerifyBlockRoot::True,
             &chain.spec,
         ) {
             match err {