!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/scala/akka/persistence/journal/japi/SyncWriteJournal.scala

@@ -12,7 +12,7 @@ import akka.persistence.PersistentImpl
  *
  * Abstract journal, optimized for synchronous writes.
  */
-abstract class SyncWriteJournal extends AsyncReplay with SSyncWriteJournal {
+abstract class SyncWriteJournal extends AsyncReplay with SSyncWriteJournal with SyncWritePlugin {
   final def write(persistent: PersistentImpl) =
     doWrite(persistent)
 
@@ -21,28 +21,4 @@ abstract class SyncWriteJournal extends AsyncReplay with SSyncWriteJournal {
 
   final def confirm(processorId: String, sequenceNr: Long, channelId: String) =
     doConfirm(processorId, sequenceNr, channelId)
-
-  /**
-   * Plugin Java API.
-   *
-   * Synchronously writes a `persistent` message to the journal.
-   */
-  @throws(classOf[Exception])
-  def doWrite(persistent: PersistentImpl): Unit
-
-  /**
-   * Plugin Java API.
-   *
-   * Synchronously marks a `persistent` message as deleted.
-   */
-  @throws(classOf[Exception])
-  def doDelete(persistent: PersistentImpl): Unit
-
-  /**
-   * Plugin Java API.
-   *
-   * Synchronously writes a delivery confirmation to the journal.
-   */
-  @throws(classOf[Exception])
-  def doConfirm(processorId: String, sequenceNr: Long, channelId: String): Unit
 }