minimize the number of places we are calling update_pubkey_cache (#1626)

## Issue Addressed

- Resolves #1080

## Proposed Changes

- Call `update_pubkey_cache` only in the `build_all_caches` method and `get_validator_index` method. 

## Additional Info

This does reduce the number of places the cache is updated, making it simpler. But the `get_validator_index` method is used a couple times when we are iterating through the entire validator registry (or set of active validators). Before, we would only call `update_pubkey_cache` once before iterating through all validators.  So I'm not _totally_ sure this change is worth it.

beacon_node/rest_api/src/consensus.rs

@@ -92,10 +92,6 @@ pub fn post_individual_votes<T: BeaconChainTypes>(
             let mut validator_statuses = ValidatorStatuses::new(&state, spec)?;
             validator_statuses.process_attestations(&state, spec)?;
 
-            state.update_pubkey_cache().map_err(|e| {
-                ApiError::ServerError(format!("Unable to build pubkey cache: {:?}", e))
-            })?;
-
             body.pubkeys
                 .into_iter()
                 .map(|pubkey| {