Consensus context with proposer index caching (#3604)

## Issue Addressed

Closes https://github.com/sigp/lighthouse/issues/2371

## Proposed Changes

Backport some changes from `tree-states` that remove duplicated calculations of the `proposer_index`.

With this change the proposer index should be calculated only once for each block, and then plumbed through to every place it is required.

## Additional Info

In future I hope to add more data to the consensus context that is cached on a per-epoch basis, like the effective balances of validators and the base rewards.

There are some other changes to remove indexing in tests that were also useful for `tree-states` (the `tree-states` types don't implement `Index`).

beacon_node/store/src/reconstruct.rs

@@ -4,7 +4,8 @@ use crate::{Error, ItemStore, KeyValueStore};
 use itertools::{process_results, Itertools};
 use slog::info;
 use state_processing::{
-    per_block_processing, per_slot_processing, BlockSignatureStrategy, VerifyBlockRoot,
+    per_block_processing, per_slot_processing, BlockSignatureStrategy, ConsensusContext,
+    VerifyBlockRoot,
 };
 use std::sync::Arc;
 use types::{EthSpec, Hash256};
@@ -87,12 +88,16 @@ where
 
                 // Apply block.
                 if let Some(block) = block {
+                    let mut ctxt = ConsensusContext::new(block.slot())
+                        .set_current_block_root(block_root)
+                        .set_proposer_index(block.message().proposer_index());
+
                     per_block_processing(
                         &mut state,
                         &block,
-                        Some(block_root),
                         BlockSignatureStrategy::NoVerification,
                         VerifyBlockRoot::True,
+                        &mut ctxt,
                         &self.spec,
                     )
                     .map_err(HotColdDBError::BlockReplayBlockError)?;