Ideally, all domains/ips blocked by GFW should go proxy in GFW mode. However, the commonly used gfw geo file is not complete. Many
blocked domains are not included. Before fallback feature, connections with those domains will fail, which impact a user's experience.
This feature is to solve this problem. Those connections will be saved by proxy nodes, which usually succeed when trying proxy. This feature
significantly improves a user's experience when using GFW mode.