SendBatch now uses pipeline mode to prepare and describe statements

Previously, a batch with 10 unique parameterized statements executed
100 times would entail 11 network round trips. 1 for each prepare /
describe and 1 for executing them all. Now pipeline mode is used to
prepare / describe all statements in a single network round trip. So it
would only take 2 round trips.

batch_test.go

@@ -420,7 +420,7 @@ func TestConnSendBatchQueryError(t *testing.T) {
 
 		err = br.Close()
 		if pgErr, ok := err.(*pgconn.PgError); !(ok && pgErr.Code == "22012") {
-			t.Errorf("rows.Err() => %v, want error code %v", err, 22012)
+			t.Errorf("br.Close() => %v, want error code %v", err, 22012)
 		}
 
 	})