!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/SyncWritePlugin.java

@@ -0,0 +1,32 @@
+/**
+ * Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package akka.persistence.journal.japi;
+
+import akka.persistence.PersistentImpl;
+
+interface SyncWritePlugin {
+    //#sync-write-plugin-api
+    /**
+     * Plugin Java API.
+     *
+     * Synchronously writes a `persistent` message to the journal.
+     */
+    void doWrite(PersistentImpl persistent) throws Exception;
+
+    /**
+     * Plugin Java API.
+     *
+     * Synchronously marks a `persistent` message as deleted.
+     */
+    void doDelete(PersistentImpl persistent) throws Exception;
+
+    /**
+     * Plugin Java API.
+     *
+     * Synchronously writes a delivery confirmation to the journal.
+     */
+    void doConfirm(String processorId, long sequenceNr, String channelId) throws Exception;
+    //#sync-write-plugin-api
+}