!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/snapshot/SnapshotStore.scala

@@ -5,7 +5,6 @@
 package akka.persistence.snapshot
 
 import scala.concurrent.Future
-import scala.util._
 
 import akka.actor._
 import akka.pattern.pipe