avoid calling sort for SortedTagMap (Netflix#1463)

When computing the id from a tag map, avoid making the
sort call when a SortedTagMap is used. That type will
already be sorted so it is not necessary.

atlas-core/src/main/scala/com/netflix/atlas/core/model/ItemIdCalculator.scala

@@ -70,16 +70,17 @@ class ItemIdCalculator {
           iter.nextEntry()
           pos += 2
         }
+        ArrayHelper.sortPairs(pairs, length)
       case _ =>
         var pos = 0
         tags.foreachEntry { (k, v) =>
           pairs(pos) = k
           pairs(pos + 1) = v
           pos += 2
         }
+        ArrayHelper.sortPairs(pairs, length)
     }
 
-    ArrayHelper.sortPairs(pairs, length)
     pairs
   }