add BackpressureBuffer, see #3253

- also make a Write’s “ack” be a Tcp.Event (to suit pipelines)
- add stress test for BackpressureBuffer
- add it to SslTlsSupportSpec
- add it to the docs

akka-docs/rst/java/code/docs/io/japi/SimpleEchoHandler.java

@@ -13,6 +13,7 @@ import akka.actor.UntypedActor;
 import akka.event.Logging;
 import akka.event.LoggingAdapter;
 import akka.io.Tcp.ConnectionClosed;
+import akka.io.Tcp.Event;
 import akka.io.Tcp.Received;
 import akka.io.TcpMessage;
 import akka.japi.Procedure;
@@ -85,7 +86,7 @@ public class SimpleEchoHandler extends UntypedActor {
   private boolean suspended = false;
   private boolean closing = false;
   
-  private final Object ACK = new Object();
+  private final Event ACK = new Event() {};
 
   //#simple-helpers
   protected void buffer(ByteString data) {