Improve unary server stub

The Java unary stub is taking the form of an async service, where a single result is expected to return from a StreamObserver.

This can be error prone:
- You can't tell from the stub whether it's unary or streaming multiple results
- The code can easily call onNext one too many times, or not call onComplete

Unary gRPC stubs for several other stubs expects a single return value from the method. Java should follow the same convention:
https://github.com/grpc/grpc/blob/master/examples/python/helloworld/greeter_server.py
https://github.com/grpc/grpc/blob/master/examples/ruby/greeter_server.rb
https://github.com/grpc/grpc-go/blob/master/examples/helloworld/greeter_server/main.go