Avoid use of Objects.hash when there's only 1 params

- directly call #hashCode() when hashed Object seems non-null
- replace with Objects.hashCode(Object) when Nullable
- replace with Long.hashCode(long), etc. when primitive

Change-Id: I08c24ebbe94cf4162d1491209a14baf953163e41
diff --git a/core/api/src/main/java/org/onosproject/store/service/WallClockTimestamp.java b/core/api/src/main/java/org/onosproject/store/service/WallClockTimestamp.java
index 0cc7b45..4be7ede 100644
--- a/core/api/src/main/java/org/onosproject/store/service/WallClockTimestamp.java
+++ b/core/api/src/main/java/org/onosproject/store/service/WallClockTimestamp.java
@@ -52,7 +52,7 @@
     }
     @Override
     public int hashCode() {
-        return Objects.hash(unixTimestamp);
+        return Long.hashCode(unixTimestamp);
     }
 
     @Override