RpcCallHandler is dispatched on the IO thread

The `RpcCallHandler` is not assigned a thread pool, which means only a single request can be handled concurrently. This is a show stopper for high throughput systems.

To process the request asynchronously, use the `addAfter` overload that takes an `EventExecutorGroup`:

https://netty.io/4.0/api/io/netty/channel/ChannelPipeline.html#addAfter-io.netty.util.concurrent.EventExecutorGroup-java.lang.String-java.lang.String-io.netty.channel.ChannelHandler-