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`).

lcli/src/skip_slots.rs

@@ -121,7 +121,7 @@ pub fn run<T: EthSpec>(mut env: Environment<T>, matches: &ArgMatches) -> Result<
     };
 
     for i in 0..runs {
-        let mut state = state.clone_with(CloneConfig::committee_caches_only());
+        let mut state = state.clone_with(CloneConfig::all());
 
         let start = Instant::now();