Add deletion of retreival transaction

If the download transaction is not deleted, the file stays open (and will cause a crash of the test).

test/test_clinkahead.cpp

@@ -628,4 +628,7 @@ TEST_F(test_clinkahead, test_up_n_download_file) {
   return_code =
     linkahead_transaction_transaction_get_result_set(&retrieve_transaction_1, &result_set);
   EXPECT_EQ(return_code, 0);
+
+  return_code = linkahead_transaction_delete_transaction(&retrieve_transaction_1);
+  EXPECT_EQ(return_code, 0);
 }