Add support for ping command in Pub/Sub mode

[sagaxu]

It's nice to use ping command to check if the connection is still alive.

• It almost works currently, except
• RedisConnection.toReceiveChannel is broken.

https://github.com/vert-x3/vertx-redis-client/blob/e3731a3e8a4d61beb4d2e03a223621a7c7252af2/src/main/java/io/vertx/redis/client/impl/RedisStandaloneConnection.java#L378

1. run in pubsub mode
2. call conn.toReceiveChannel, it will pause the stream first, then call ReadStream.fetch(1)
3. run ping command and redis server reply with pong
4. the handle in RedisStandaloneConnection didn't dispatch the reply of ping to onMessage
5. so nobody will call stream.fetch(1), the stream can't receive any more

private class ChannelReadStream<T>(val stream: ReadStream<T>,
                                   val channel: Channel<T>,
                                   context: Context) : Channel<T> by channel, CoroutineScope {

  override val coroutineContext: CoroutineContext = context.dispatcher()

  fun subscribe() {
    stream.endHandler {
      close()
    }
    stream.exceptionHandler { err ->
      close(err)
    }
    stream.handler { event ->
      launch {
        send(event)
        stream.fetch(1)
      }
    }
  }
}