ARROW-7546: [Java] Use new implementation to concat vectors values in batch

Related to [ARROW-7546](https://issues.apache.org/jira/browse/ARROW-7546).

Per discussion apache/arrow#5945 (comment).

In ARROW-7284, we write a simple method to concat vectors. However, ARROW-7073 is about to concat vector values efficiently, after this PR merged, we should use this new implementation in ArrowReader.

Closes #6431 from tianchen92/ARROW-7546 and squashes the following commits:

9bced461c <tianchen> ARROW-7546:  Use new implementation to concat vectors values in batch

Authored-by: tianchen <[email protected]>
Signed-off-by: Micah Kornfield <[email protected]>

Author: tianchen92

vector/src/main/java/org/apache/arrow/vector/ipc/ArrowReader.java

@@ -35,7 +35,7 @@
 import org.apache.arrow.vector.types.pojo.Field;
 import org.apache.arrow.vector.types.pojo.Schema;
 import org.apache.arrow.vector.util.DictionaryUtility;
-import org.apache.arrow.vector.util.TransferPair;
+import org.apache.arrow.vector.util.VectorBatchAppender;
 
 /**
  * Abstract class to read Schema and ArrowRecordBatches.
@@ -224,7 +224,8 @@ protected void loadDictionary(ArrowDictionaryBatch dictionaryBatch) {
     if (dictionaryBatch.isDelta()) {
       FieldVector deltaVector = vector.getField().createVector(allocator);
       load(dictionaryBatch, deltaVector);
-      concatDeltaDictionary(vector, deltaVector);
+      VectorBatchAppender.batchAppend(vector, deltaVector);
+      deltaVector.close();
       return;
     }
 
@@ -242,18 +243,4 @@ private void load(ArrowDictionaryBatch dictionaryBatch, FieldVector vector) {
       dictionaryBatch.close();
     }
   }
-
-  /**
-   * Concat dictionary vector and delta dictionary vector.
-   */
-  private void concatDeltaDictionary(FieldVector vector, FieldVector deltaVector) {
-    final int valueCount = vector.getValueCount();
-    final int deltaValueCount = deltaVector.getValueCount();
-    final TransferPair transferPair = deltaVector.makeTransferPair(vector);
-    for (int i = 0; i < deltaValueCount; i++) {
-      transferPair.copyValueSafe(i, valueCount + i);
-    }
-    deltaVector.close();
-    vector.setValueCount(valueCount + deltaValueCount);
-  }
 }