changing persistence engine trait to an abstract class

Author: niranda <niranda.perera@gmail.com>

Closes #5832 from nirandaperera/PersistanceEngine_abstract_class and squashes the following commits:

67b9d5a [niranda] changing persistence engine trait to an abstract class

core/src/main/scala/org/apache/spark/deploy/master/PersistenceEngine.scala

@@ -33,7 +33,7 @@ import scala.reflect.ClassTag
  * The implementation of this trait defines how name-object pairs are stored or retrieved.
  */
 @DeveloperApi
-trait PersistenceEngine {
+abstract class PersistenceEngine {
 
   /**
    * Defines how the object is serialized and persisted. Implementation will