!per #3652 Custom snapshot and persistent message serialization

- Protobuf serializer for Persistent message
- Configurable serializer for Persistent message's payload
- Configurable serializer for snapshots

akka-persistence/src/main/java/akka/persistence/journal/japi/AsyncWritePlugin.java

@@ -0,0 +1,34 @@
+/**
+ * Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package akka.persistence.journal.japi;
+
+import scala.concurrent.Future;
+
+import akka.persistence.PersistentImpl;
+
+interface AsyncWritePlugin {
+    //#async-write-plugin-api
+    /**
+     * Plugin Java API.
+     *
+     * Asynchronously writes a `persistent` message to the journal.
+     */
+    Future<Void> doWriteAsync(PersistentImpl persistent);
+
+    /**
+     * Plugin Java API.
+     *
+     * Asynchronously marks a `persistent` message as deleted.
+     */
+    Future<Void> doDeleteAsync(PersistentImpl persistent);
+
+    /**
+     * Plugin Java API.
+     *
+     * Asynchronously writes a delivery confirmation to the journal.
+     */
+    Future<Void> doConfirmAsync(String processorId, long sequenceNr, String channelId);
+    //#async-write-plugin-api
+}