[SQL] [Minor] Update the SpecificMutableRow.copy

When profiling the Join / Aggregate queries via VisualVM, I noticed lots of `SpecificMutableRow` objects created, as well as the `MutableValue`, since the `SpecificMutableRow` are mostly used in data source implementation, but the `copy` method could be called multiple times in upper modules (e.g. in Join / aggregation etc.), duplicated instances created should be avoid.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #4619 from chenghao-intel/specific_mutable_row and squashes the following commits:

9300d23 [Cheng Hao] update the SpecificMutableRow.copy

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/SpecificMutableRow.scala

@@ -220,13 +220,14 @@ final class SpecificMutableRow(val values: Array[MutableValue]) extends MutableR
   override def isNullAt(i: Int): Boolean = values(i).isNull
 
   override def copy(): Row = {
-    val newValues = new Array[MutableValue](values.length)
+    val newValues = new Array[Any](values.length)
     var i = 0
     while (i < values.length) {
-      newValues(i) = values(i).copy()
+      newValues(i) = values(i).boxed
       i += 1
     }
-    new SpecificMutableRow(newValues)
+
+    new GenericRow(newValues)
   }
 
   override def update(ordinal: Int, value: Any): Unit = {