!per #3707 Channel enhancements

- Persistent channel
- ConfirmablePersistent message type delivered by channel
- Sender resolution performance improvements
   * unstash() instead of unstashAll()

These enhancements required the following changes

- Unified implementation of processor stash and user stash
- Persistence message plugin API separated from implementation
- Physical deletion of messages

akka-persistence/src/main/scala/akka/persistence/serialization/SnapshotSerializer.scala

@@ -1,4 +1,5 @@
 /**
+ * Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
  * Copyright (C) 2012-2013 Eligotech BV.
  */