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

consensus/state_processing/src/per_block_processing/tests.rs

@@ -6,7 +6,10 @@ use crate::per_block_processing::errors::{
     DepositInvalid, HeaderInvalid, IndexedAttestationInvalid, IntoWithIndex,
     ProposerSlashingInvalid,
 };
-use crate::{per_block_processing::process_operations, BlockSignatureStrategy, VerifySignatures};
+use crate::{
+    per_block_processing::process_operations, BlockSignatureStrategy, VerifyBlockRoot,
+    VerifySignatures,
+};
 use beacon_chain::test_utils::{BeaconChainHarness, EphemeralHarnessType};
 use lazy_static::lazy_static;
 use ssz_types::Bitfield;
@@ -65,6 +68,7 @@ fn valid_block_ok() {
         &block,
         None,
         BlockSignatureStrategy::VerifyIndividual,
+        VerifyBlockRoot::True,
         &spec,
     );
 
@@ -88,6 +92,7 @@ fn invalid_block_header_state_slot() {
         &SignedBeaconBlock::from_block(block, signature),
         None,
         BlockSignatureStrategy::VerifyIndividual,
+        VerifyBlockRoot::True,
         &spec,
     );
 
@@ -116,6 +121,7 @@ fn invalid_parent_block_root() {
         &SignedBeaconBlock::from_block(block, signature),
         None,
         BlockSignatureStrategy::VerifyIndividual,
+        VerifyBlockRoot::True,
         &spec,
     );
 
@@ -145,6 +151,7 @@ fn invalid_block_signature() {
         &SignedBeaconBlock::from_block(block, Signature::empty()),
         None,
         BlockSignatureStrategy::VerifyIndividual,
+        VerifyBlockRoot::True,
         &spec,
     );
 
@@ -174,6 +181,7 @@ fn invalid_randao_reveal_signature() {
         &signed_block,
         None,
         BlockSignatureStrategy::VerifyIndividual,
+        VerifyBlockRoot::True,
         &spec,
     );