Override Timestamp hashCode for consistent hashing

Timestamp's hashCode was based on the parameter dataTime which might have
a timezone != UTC. The consequence:

  val t = Timestamp.now
  t.hashCode != Timestamp(t.toString).hashCode

This patch overrides hashCode to use the UTC normalized time variable of the
Timestamp case class.

This probably fixes d2iq-archive#1082

src/main/scala/mesosphere/marathon/state/Timestamp.scala

@@ -15,6 +15,8 @@ case class Timestamp(dateTime: DateTime) extends Ordered[Timestamp] {
     case _               => false
   }
 
+  override def hashCode: Int = time.hashCode
+
   def compare(that: Timestamp): Int = this.time compareTo that.time
 
   override def toString: String = time.toString