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/lib.rs

@@ -16,6 +16,7 @@
 mod macros;
 mod metrics;
 
+pub mod block_replayer;
 pub mod common;
 pub mod genesis;
 pub mod per_block_processing;
@@ -25,13 +26,14 @@ pub mod state_advance;
 pub mod upgrade;
 pub mod verify_operation;
 
+pub use block_replayer::{BlockReplayError, BlockReplayer, StateRootStrategy};
 pub use genesis::{
     eth2_genesis_time, initialize_beacon_state_from_eth1, is_valid_genesis_state,
     process_activations,
 };
 pub use per_block_processing::{
     block_signature_verifier, errors::BlockProcessingError, per_block_processing, signature_sets,
-    BlockSignatureStrategy, BlockSignatureVerifier, VerifySignatures,
+    BlockSignatureStrategy, BlockSignatureVerifier, VerifyBlockRoot, VerifySignatures,
 };
 pub use per_epoch_processing::{
     errors::EpochProcessingError, process_epoch as per_epoch_processing,