[MP] Partially revert cache cleanup, track paths as fallback

Cleaning up remote NodePath cache is not trivial since the visibility
API allows for certain nodes to be despawned (and re-spawned) on some
peers while being retained in the authority.

This means that from the server point of view, the node has not changed,
and the path simplification protocol won't be run again after
respawning.

While we can track this information for synchronizers via the
replication API, we can't easily track this information for potential
child nodes that use RPCs (I'm convinced it is doable, but we need to
track the whole dependency tree which would require some more complex
refactoring).

This commit partially reverts some of the cache cleanup logic to always
retain remote IDs, and adds a NodePath lookup fallback when the ObjectID
is invalid.

modules/multiplayer/scene_cache_interface.h

@@ -49,8 +49,18 @@ private:
 		HashMap<int, bool> confirmed_peers; // peer id, confirmed
 	};
 
+	struct RecvNode {
+		ObjectID oid;
+		NodePath path;
+
+		RecvNode(const ObjectID &p_oid, const NodePath &p_path) {
+			oid = p_oid;
+			path = p_path;
+		}
+	};
+
 	struct PeerInfo {
-		HashMap<int, ObjectID> recv_nodes; // remote cache id, ObjectID
+		HashMap<int, RecvNode> recv_nodes; // remote cache id, (ObjectID, NodePath)
 		HashSet<ObjectID> sent_nodes;
 	};