Add early check for RPC block relevancy (#2289)

## Issue Addressed

NA

## Proposed Changes

When observing `jemallocator` heap profiles and Grafana, it became clear that Lighthouse is spending significant RAM/CPU on processing blocks from the RPC. On investigation, it seems that we are loading the parent of the block *before* we check to see if the block is already known. This is a big waste of resources.

This PR adds an additional `check_block_relevancy` call as the first thing we do when we try to process a `SignedBeaconBlock` via the RPC (or other similar methods). Ultimately, `check_block_relevancy` will be called again later in the block processing flow. It's a very light function and I don't think trying to optimize it out is worth the risk of a bad block slipping through. 

Also adds a `New RPC block received` info log when we process a new RPC block. This seems like interesting and infrequent info.

## Additional Info

NA

beacon_node/network/src/beacon_processor/worker/sync_methods.rs

@@ -6,7 +6,7 @@ use crate::sync::manager::SyncMessage;
 use crate::sync::{BatchProcessResult, ChainId};
 use beacon_chain::{BeaconChainTypes, BlockError, ChainSegmentResult};
 use eth2_libp2p::PeerId;
-use slog::{crit, debug, error, trace, warn};
+use slog::{crit, debug, error, info, trace, warn};
 use types::{Epoch, Hash256, SignedBeaconBlock};
 
 /// Id associated to a block processing request, either a batch or a single block.
@@ -28,10 +28,20 @@ impl<T: BeaconChainTypes> Worker<T> {
         block: SignedBeaconBlock<T::EthSpec>,
         result_tx: BlockResultSender<T::EthSpec>,
     ) {
+        let slot = block.slot();
         let block_result = self.chain.process_block(block);
 
         metrics::inc_counter(&metrics::BEACON_PROCESSOR_RPC_BLOCK_IMPORTED_TOTAL);
 
+        if let Ok(root) = &block_result {
+            info!(
+                self.log,
+                "New RPC block received";
+                "slot" => slot,
+                "hash" => %root
+            );
+        }
+
         if result_tx.send(block_result).is_err() {
             crit!(self.log, "Failed return sync block result");
         }