Refined payload pruning (#3587)

## Proposed Changes

Improve the payload pruning feature in several ways:

- Payload pruning is now entirely optional. It is enabled by default but can be disabled with `--prune-payloads false`. The previous `--prune-payloads-on-startup` flag from #3565 is removed.
- Initial payload pruning on startup now runs in a background thread. This thread will always load the split state, which is a small fraction of its total work (up to ~300ms) and then backtrack from that state. This pruning process ran in 2m5s on one Prater node with good I/O and 16m on a node with slower I/O.
- To work with the optional payload pruning the database function `try_load_full_block` will now attempt to load execution payloads for finalized slots _if_ pruning is currently disabled. This gives users an opt-out for the extensive traffic between the CL and EL for reconstructing payloads.

## Additional Info

If the `prune-payloads` flag is toggled on and off then the on-startup check may not see any payloads to delete and fail to clean them up. In this case the `lighthouse db prune_payloads` command should be used to force a manual sweep of the database.

beacon_node/beacon_chain/src/builder.rs

@@ -266,13 +266,6 @@ where
 
         self.genesis_time = Some(genesis_state.genesis_time());
 
-        // Prune finalized execution payloads.
-        if store.get_config().prune_payloads_on_init {
-            store
-                .try_prune_execution_payloads(false)
-                .map_err(|e| format!("Error pruning execution payloads: {e:?}"))?;
-        }
-
         self.op_pool = Some(
             store
                 .get_item::<PersistedOperationPool<TEthSpec>>(&OP_POOL_DB_KEY)
@@ -863,6 +856,20 @@ where
             beacon_chain.store_migrator.process_reconstruction();
         }
 
+        // Prune finalized execution payloads in the background.
+        if beacon_chain.store.get_config().prune_payloads {
+            let store = beacon_chain.store.clone();
+            let log = log.clone();
+            beacon_chain.task_executor.spawn_blocking(
+                move || {
+                    if let Err(e) = store.try_prune_execution_payloads(false) {
+                        error!(log, "Error pruning payloads in background"; "error" => ?e);
+                    }
+                },
+                "prune_payloads_background",
+            );
+        }
+
         Ok(beacon_chain)
     }
 }