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

testing/state_transition_vectors/src/exit.rs

@@ -2,7 +2,7 @@ use super::*;
 use beacon_chain::test_utils::{BeaconChainHarness, EphemeralHarnessType};
 use state_processing::{
     per_block_processing, per_block_processing::errors::ExitInvalid, BlockProcessingError,
-    BlockSignatureStrategy,
+    BlockSignatureStrategy, VerifyBlockRoot,
 };
 use types::{BeaconBlock, BeaconState, Epoch, EthSpec, SignedBeaconBlock};
 
@@ -66,6 +66,7 @@ impl ExitTest {
             block,
             None,
             BlockSignatureStrategy::VerifyIndividual,
+            VerifyBlockRoot::True,
             &E::default_spec(),
         )
     }