示例：https://github.com/aspnet/SignalR-samples
中心协议规范：https://github.com/dotnet/aspnetcore/blob/master/src/SignalR/docs/specs/HubProtocol.md
传输协议规范：https://github.com/dotnet/aspnetcore/blob/master/src/SignalR/docs/specs/TransportProtocols.md
