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.

consensus/state_processing/src/per_block_processing/verify_deposit.rs

@@ -35,7 +35,7 @@ pub fn verify_deposit_signature(deposit_data: &DepositData, spec: &ChainSpec) ->
 ///
 /// Errors if the state's `pubkey_cache` is not current.
 pub fn get_existing_validator_index<T: EthSpec>(
-    state: &BeaconState<T>,
+    state: &mut BeaconState<T>,
     pub_key: &PublicKeyBytes,
 ) -> Result<Option<u64>> {
     let validator_index = state.get_validator_index(pub_key)?;