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/store/src/errors.rs

@@ -2,6 +2,7 @@ use crate::chunked_vector::ChunkError;
 use crate::config::StoreConfigError;
 use crate::hot_cold_store::HotColdDBError;
 use ssz::DecodeError;
+use state_processing::BlockReplayError;
 use types::{BeaconStateError, Hash256, Slot};
 
 pub type Result<T> = std::result::Result<T, Error>;
@@ -39,6 +40,7 @@ pub enum Error {
         expected: Hash256,
         computed: Hash256,
     },
+    BlockReplayError(BlockReplayError),
 }
 
 pub trait HandleUnavailable<T> {
@@ -91,6 +93,12 @@ impl From<StoreConfigError> for Error {
     }
 }
 
+impl From<BlockReplayError> for Error {
+    fn from(e: BlockReplayError) -> Error {
+        Error::BlockReplayError(e)
+    }
+}
+
 #[derive(Debug)]
 pub struct DBError {
     pub message: String,