Check lateness of block before requeuing it (#4208)

## Issue Addressed

NA

## Proposed Changes

Avoids reprocessing loops introduced in #4179. (Also somewhat related to #4192).

Breaks the re-queue loop by only re-queuing when an RPC block is received before the attestation creation deadline.

I've put `proposal_is_known` behind a closure to avoid interacting with the `observed_proposers` lock unnecessarily. 

## Additional Info

NA

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

@@ -9,12 +9,15 @@ use crate::sync::manager::{BlockProcessType, SyncMessage};
 use crate::sync::{BatchProcessResult, ChainId};
 use beacon_chain::CountUnrealized;
 use beacon_chain::{
-    observed_block_producers::Error as ObserveError, BeaconChainError, BeaconChainTypes,
+    observed_block_producers::Error as ObserveError, validator_monitor::get_block_delay_ms,
-    BlockError, ChainSegmentResult, HistoricalBlockError, NotifyExecutionLayer,
+    BeaconChainError, BeaconChainTypes, BlockError, ChainSegmentResult, HistoricalBlockError,
+    NotifyExecutionLayer,
 };
 use lighthouse_network::PeerAction;
 use slog::{debug, error, info, warn};
+use slot_clock::SlotClock;
 use std::sync::Arc;
+use std::time::{SystemTime, UNIX_EPOCH};
 use tokio::sync::mpsc;
 use types::{Epoch, Hash256, SignedBeaconBlock};
 
@@ -83,9 +86,21 @@ impl<T: BeaconChainTypes> Worker<T> {
                 return;
             }
         };
-        // Check if a block from this proposer is already known. If so, defer processing until later
+
-        // to avoid wasting time processing duplicates.
+        // Returns `true` if the time now is after the 4s attestation deadline.
-        let proposal_already_known = match self
+        let block_is_late = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            // If we can't read the system time clock then indicate that the
+            // block is late (and therefore should *not* be requeued). This
+            // avoids infinite loops.
+            .map_or(true, |now| {
+                get_block_delay_ms(now, block.message(), &self.chain.slot_clock)
+                    > self.chain.slot_clock.unagg_attestation_production_delay()
+            });
+
+        // Checks if a block from this proposer is already known.
+        let proposal_already_known = || {
+            match self
                 .chain
                 .observed_block_producers
                 .read()
@@ -96,8 +111,13 @@ impl<T: BeaconChainTypes> Worker<T> {
                 // than re-queuing them.
                 Err(ObserveError::FinalizedBlock { .. })
                 | Err(ObserveError::ValidatorIndexTooHigh { .. }) => false,
+            }
         };
-        if proposal_already_known {
+
+        // If we've already seen a block from this proposer *and* the block
+        // arrived before the attestation deadline, requeue it to ensure it is
+        // imported late enough that it won't receive a proposer boost.
+        if !block_is_late && proposal_already_known() {
             debug!(
                 self.log,
                 "Delaying processing of duplicate RPC block";