Add code generation for the advanced server interface

I asked on the mailing list (https://groups.google.com/forum/#!topic/grpc-io/1RiPC8XYP_U) whether a callback existed to notify the server that a client had disconnected so it could perform some cleanup actions. Here's the answer I received from Eric Anderson:

Because it is a unary request, there isn't a callback that notifies you of client cancellation (any error is considered a cancellation to the server). The next time you call onValue though, an exception will be thrown notifying you of the cancellation. That is enough in some scenarios.

The "advanced" interface provides such notification though. It has onHalfClose (which is when onComplete is normally called), onComplete (when the RPC gracefully completed), and onCancel (if there was an error or if the client cancelled). We don't have code generation for the advanced interface though, so you would need to implement ServerCallHandler for each method in the service, create ServerMethodDefinitions and add them to a ServerServiceDefinition, which is what you pass to the ServerBuilder's addService. That would be verbose and annoying, but simple code. It would probably make sense for you to create an issue for us to add codegen for the advanced interface.

首先阅读高级接口，以及手动描述的 ServerCallHandler、ServerMethodDefinitions、ServerServiceDefinition 和 ServerBuilder.addService 流程。将其与现有的生成式 service 路径进行比较，然后定义生成的 output 必须为 onHalfClose、onComplete 和 onCancel 提供什么；完成的标准是可以通过代码生成使用高级接口，而不是为每个 method 手动设置。

由索引模型根据 Issue 内容生成。