Artefact last modified is returned my DMF auth call (#600)

* Last modified is stored as part of artifact and returns accordingly by
APIs.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Fixed tests.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Check value > 0

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Use created at.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Sonar issue fixed.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

hawkbit-repository/hawkbit-repository-api/src/main/java/org/eclipse/hawkbit/repository/model/BaseEntity.java

@@ -27,7 +27,7 @@ public interface BaseEntity extends Serializable, Identifiable<Long> {
      * @return time in {@link TimeUnit#MILLISECONDS} when the {@link BaseEntity}
      *         was created.
      */
-    Long getCreatedAt();
+    long getCreatedAt();
 
     /**
      * @return user that created the {@link BaseEntity}.
@@ -38,7 +38,7 @@ public interface BaseEntity extends Serializable, Identifiable<Long> {
      * @return time in {@link TimeUnit#MILLISECONDS} when the {@link BaseEntity}
      *         was last time changed.
      */
-    Long getLastModifiedAt();
+    long getLastModifiedAt();
 
     /**
      * @return user that updated the {@link BaseEntity} last.