[MP] Gracefully handle cache confirmation of deleted nodes

It's possible that after sending a cached node reference (e.g. RPC or
static MultiplayerSynchronizer) the reference node is removed from tree
before the remote peer(s) can confirm the referenced path.

To better detect that case, and avoid spamming errors when it happens,
this commit modifies the multiplayer API caching protocol, to send the
received ID instead of the Node path when sending the confirmation
packet.

**This is a breaking change** because it makes the runtime multiplayer
protocol incompatible with previous versions of Godot.

modules/multiplayer/scene_cache_interface.h

@@ -44,7 +44,7 @@ private:
 
 	//path sent caches
 	struct NodeCache {
-		int cache_id;
+		int cache_id = 0;
 		HashMap<int, int> recv_ids; // peer id, remote cache id
 		HashMap<int, bool> confirmed_peers; // peer id, confirmed
 	};
@@ -55,6 +55,7 @@ private:
 	};
 
 	HashMap<ObjectID, NodeCache> nodes_cache;
+	HashMap<int, ObjectID> assigned_ids;
 	HashMap<int, PeerInfo> peers_info;
 	int last_send_cache_id = 1;