Keep payload cache idempotent (#4256)

## Issue Addressed

[#4239](https://github.com/sigp/lighthouse/issues/4239)

## Proposed Changes

keep the payload cache entry intact after fetching it

## Additional Info

beacon_node/execution_layer/src/payload_cache.rs

@@ -30,4 +30,8 @@ impl<T: EthSpec> PayloadCache<T> {
     pub fn pop(&self, root: &Hash256) -> Option<ExecutionPayload<T>> {
         self.payloads.lock().pop(&PayloadCacheId(*root))
     }
+
+    pub fn get(&self, hash: &Hash256) -> Option<ExecutionPayload<T>> {
+        self.payloads.lock().get(&PayloadCacheId(*hash)).cloned()
+    }
 }