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.

consensus/state_processing/src/per_block_processing.rs

@@ -46,6 +46,8 @@ pub enum BlockSignatureStrategy {
     NoVerification,
     /// Validate each signature individually, as its object is being processed.
     VerifyIndividual,
+    /// Validate only the randao reveal signature.
+    VerifyRandao,
     /// Verify all signatures in bulk at the beginning of block processing.
     VerifyBulk,
 }
@@ -115,6 +117,7 @@ pub fn per_block_processing<T: EthSpec>(
         }
         BlockSignatureStrategy::VerifyIndividual => VerifySignatures::True,
         BlockSignatureStrategy::NoVerification => VerifySignatures::False,
+        BlockSignatureStrategy::VerifyRandao => VerifySignatures::False,
     };
 
     let proposer_index = process_block_header(state, block, spec)?;
@@ -123,11 +126,16 @@ pub fn per_block_processing<T: EthSpec>(
         verify_block_signature(state, signed_block, block_root, spec)?;
     }
 
+    let verify_randao = if let BlockSignatureStrategy::VerifyRandao = block_signature_strategy {
+        VerifySignatures::True
+    } else {
+        verify_signatures
+    };
     // Ensure the current and previous epoch caches are built.
     state.build_committee_cache(RelativeEpoch::Previous, spec)?;
     state.build_committee_cache(RelativeEpoch::Current, spec)?;
 
-    process_randao(state, block, verify_signatures, spec)?;
+    process_randao(state, block, verify_randao, spec)?;
     process_eth1_data(state, block.body().eth1_data())?;
     process_operations(state, block.body(), proposer_index, verify_signatures, spec)?;