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/errors.rs

@@ -20,7 +20,7 @@ use state_processing::{
     },
     signature_sets::Error as SignatureSetError,
     state_advance::Error as StateAdvanceError,
-    BlockProcessingError, SlotProcessingError,
+    BlockProcessingError, BlockReplayError, SlotProcessingError,
 };
 use std::time::Duration;
 use task_executor::ShutdownReason;
@@ -86,6 +86,7 @@ pub enum BeaconChainError {
     ValidatorPubkeyCacheIncomplete(usize),
     SignatureSetError(SignatureSetError),
     BlockSignatureVerifierError(state_processing::block_signature_verifier::Error),
+    BlockReplayError(BlockReplayError),
     DuplicateValidatorPublicKey,
     ValidatorPubkeyCacheFileError(String),
     ValidatorIndexUnknown(usize),
@@ -160,6 +161,7 @@ easy_from_to!(ArithError, BeaconChainError);
 easy_from_to!(ForkChoiceStoreError, BeaconChainError);
 easy_from_to!(HistoricalBlockError, BeaconChainError);
 easy_from_to!(StateAdvanceError, BeaconChainError);
+easy_from_to!(BlockReplayError, BeaconChainError);
 
 #[derive(Debug)]
 pub enum BlockProductionError {