Improve client API for client streaming.

[jcornaz]

Reading https://github.com/rouzwawi/grpc-kotlin#client I'm a bit surprised that the streaming from client to server are done by calling send on the result of the function.

Personally I'd rather make the function taking a ReceiveChannel.

Here is how the example in readme would look like:

// === Client streaming call ==================================================================

coroutineScope {
  val oneReply = greeter.greetClientStream(produce { // <-- greetClientStream would suspend
    send(req("Caroline"))
    send(req("David"))
  })
  println("single reply = ${oneReply.reply}")
}

// === Bidirectional call =====================================================================

coroutineScope {
  val responses = greeter.greetBidirectional(produce {
  	delay(200)
  	send(req("Eve"))
  
  	delay(200)
  	send(req("Fred"))
  
  	delay(200)
  	send(req("Gina"))
  })
  
  launch {
    var n = 0
    for (greetReply in responses) {
      println("r$n = ${greetReply.reply}")
      n++
    }
    println("no more replies")
  }
}

Advantages of this style:

• It is more consistent with how it looks like on the server side.
• Client streaming call is more consistent with Unary call, since they both suspend. (Which make sense, because both return a value and not a channel. See Function returning ReceiveChannel should not be marked with suspend #30)
• It is safer because:
  • it is not possible to forget to close() the stream.
  • it is not possible to forget to call await().
  • thanks to coroutineScope we get automatic cancellation of all streams in case of failure of one job.
• Bi directional API is less confusing and provide a clearer separation of the outgoing and incoming streams. (currently the same object is used to send and receive elements)