Sink didn't pull the first item

[ticofab]

I couldn't get the RSocketFFSink to work, until I added a first pull after the RSocket connection has been established:

// ask for an element as soon as you start
override def preStart(): Unit = {
  println(s"pre start, connecting to $host:$port")
  ...
  pull(in)
}

After adding this, everything works as expected and new items are pulled one by one after the previous one has been processed.