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/beacon.rs

@@ -192,12 +192,11 @@ pub fn get_all_validators<T: BeaconChainTypes>(
     };
 
     let mut state = get_state_from_root_opt(&ctx.beacon_chain, state_root_opt)?;
-    state.update_pubkey_cache()?;
 
-    state
-        .validators
+    let validators = state.validators.clone();
+    validators
         .iter()
-        .map(|validator| validator_response_by_pubkey(&state, validator.pubkey.clone()))
+        .map(|validator| validator_response_by_pubkey(&mut state, validator.pubkey.clone()))
         .collect::<Result<Vec<_>, _>>()
 }
 
@@ -215,13 +214,14 @@ pub fn get_active_validators<T: BeaconChainTypes>(
     };
 
     let mut state = get_state_from_root_opt(&ctx.beacon_chain, state_root_opt)?;
-    state.update_pubkey_cache()?;
 
-    state
-        .validators
+    let validators = state.validators.clone();
+    let current_epoch = state.current_epoch();
+
+    validators
         .iter()
-        .filter(|validator| validator.is_active_at(state.current_epoch()))
-        .map(|validator| validator_response_by_pubkey(&state, validator.pubkey.clone()))
+        .filter(|validator| validator.is_active_at(current_epoch))
+        .map(|validator| validator_response_by_pubkey(&mut state, validator.pubkey.clone()))
         .collect::<Result<Vec<_>, _>>()
 }
 
@@ -279,11 +279,10 @@ fn validator_responses_by_pubkey<T: BeaconChainTypes>(
     validator_pubkeys: Vec<PublicKeyBytes>,
 ) -> Result<Vec<ValidatorResponse>, ApiError> {
     let mut state = get_state_from_root_opt(beacon_chain, state_root_opt)?;
-    state.update_pubkey_cache()?;
 
     validator_pubkeys
         .into_iter()
-        .map(|validator_pubkey| validator_response_by_pubkey(&state, validator_pubkey))
+        .map(|validator_pubkey| validator_response_by_pubkey(&mut state, validator_pubkey))
         .collect::<Result<Vec<_>, ApiError>>()
 }
 
@@ -291,7 +290,7 @@ fn validator_responses_by_pubkey<T: BeaconChainTypes>(
 ///
 /// The provided `state` must have a fully up-to-date pubkey cache.
 fn validator_response_by_pubkey<E: EthSpec>(
-    state: &BeaconState<E>,
+    state: &mut BeaconState<E>,
     validator_pubkey: PublicKeyBytes,
 ) -> Result<ValidatorResponse, ApiError> {
     let validator_index_opt = state

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| {

beacon_node/rest_api/src/validator.rs

@@ -156,9 +156,6 @@ fn return_validator_duties<T: BeaconChainTypes>(
     state
         .build_committee_cache(relative_epoch, &beacon_chain.spec)
         .map_err(|e| ApiError::ServerError(format!("Unable to build committee cache: {:?}", e)))?;
-    state
-        .update_pubkey_cache()
-        .map_err(|e| ApiError::ServerError(format!("Unable to build pubkey cache: {:?}", e)))?;
 
     // Get a list of all validators for this epoch.
     //