RMQSource is not parallel

The official documentation states that RMQSource can have at-least-once processing when "when checkpointing is enabled, but correlation ids are not used or the source is parallel." However, as RMQSource does not implement ParallelSourceFunction, StreamExecutionEnvironment will throw an exception whenever parallelism is set to a value higher than 1.

Either the documentation should be corrected or RMQSource should implement ParallelSourceFunction.

Implementing it seems like it should be fairly easy and I'd be willing to submit a patch, but as I'm new to the Flink code base I'm not sure if there would be unexpected side effects. If I can get feedback from more experienced contributors, I'll submit a patch in the near future.