add reactive-stream support

https://github.com/netty/netty/issues/3902

I think it would be nice.

I am currently using reactive-kafka to consume protobuf message,and then pumping it to graylog.

```
message LoggingEvent {
required LoggingLevel level = 1;
required string host = 2 [default = "localhost"];
optional string source = 3;
optional string thread = 4;
optional string logClass = 5;
optional int64 timestamp = 6;
message LoggingMDCEntity{
required string key = 1;
required string value =2;
}
repeated LoggingMDCEntity mdc =7;
optional string message = 8;
optional string stackTrace = 9;
}

enum LoggingLevel {
ERROR = 1;
WARNING = 2;
INFO = 3;
DEBUG = 4;
}
```