improve DirectByteBufferPool and some smaller fixes

* dispatch Closed events to the sender of the close command as well
 * DirectByteBufferPool changes:

     * hide behind BufferPool interface
     * only return buffers of the configured size
     * reduce work to be done while locked
     * use Array-based stack to store free buffers instead of linked lists
     * keep buffers by soft reference

akka-io/src/test/scala/akka/io/TcpConnectionSpec.scala

@@ -14,7 +14,7 @@ import java.net._
 import scala.collection.immutable
 import scala.concurrent.duration._
 import scala.util.control.NonFatal
-import akka.actor.{ ActorRef, Terminated }
+import akka.actor.{ PoisonPill, ActorRef, Terminated }
 import akka.testkit.{ TestProbe, TestActorRef, AkkaSpec }
 import akka.util.ByteString
 import TestUtils._
@@ -84,7 +84,7 @@ class TcpConnectionSpec extends AkkaSpec("akka.io.tcp.register-timeout = 500ms")
       buffer.clear()
       serverSideChannel.read(buffer) must be(0)
       writer.send(connectionActor, unackedWrite)
-      writer.expectNoMsg()
+      writer.expectNoMsg(500.millis)
       serverSideChannel.read(buffer) must be(10)
       buffer.flip()
       ByteString(buffer).take(10).decodeString("ASCII") must be("morestuff!")
@@ -172,7 +172,7 @@ class TcpConnectionSpec extends AkkaSpec("akka.io.tcp.register-timeout = 500ms")
 
       connectionHandler.send(connectionActor, Close)
       connectionHandler.expectMsg(Closed)
-      connectionHandler.expectNoMsg()
+      connectionHandler.expectNoMsg(500.millis)
     }
 
     "abort the connection and reply with `Aborted` upong reception of an `Abort` command" in withEstablishedConnection() { setup ⇒
@@ -293,8 +293,7 @@ class TcpConnectionSpec extends AkkaSpec("akka.io.tcp.register-timeout = 500ms")
     "close the connection when user handler dies while connecting" in withUnacceptedConnection() { setup ⇒
       import setup._
 
-      // simulate death of userHandler test probe
-      userHandler.send(connectionActor, akka.actor.Terminated(userHandler.ref)(false, false))
+      userHandler.ref ! PoisonPill
 
       verifyActorTermination(connectionActor)
     }