Check proposer index during block production (#2740)

## Issue Addressed

Resolves #2612 

## Proposed Changes

Implements both the checks mentioned in the original issue. 
1. Verifies the `randao_reveal` in the beacon node
2. Cross checks the proposer index after getting back the block from the beacon node.

## Additional info
The block production time increases by ~10x because of the signature verification on the beacon node (based on the `beacon_block_production_process_seconds` metric) when running on a local testnet.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -2889,7 +2889,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             &mut state,
             &block,
             None,
-            BlockSignatureStrategy::NoVerification,
+            BlockSignatureStrategy::VerifyRandao,
             &self.spec,
         )?;
         drop(process_timer);