Control block buffer size for performance

[gingerwizard]

Block buffer size has historically been 2. This limits the parallelization of scan and decoding. This change introduces the setting BlockBufferSize (defaults to 2). Users can increase this at the expense of memory in return for speedups on read e.g.

func TestRead(b *testing.T) {
	conn := getConnection()
	start := time.Now()
	rows, err := conn.Query(clickhouse.Context(context.Background(), clickhouse.WithBlockBufferSize(2)), fmt.Sprintf(`SELECT number FROM system.numbers_mt LIMIT 500000000`))
	if err != nil {
		b.Fatal(err)
	}
	var (
		col1 uint64
	)
	for rows.Next() {
		if err := rows.Scan(&col1); err != nil {
			b.Fatal(err)
		}
	}
	elapsed := time.Since(start)
	log.Printf("Read took %s", elapsed)
}

takes

=== RUN   TestRead
2022/10/04 11:10:12 Read took 45.440940766s

increasing to clickhouse.WithBlockBufferSize(100).

gives

=== RUN   TestRead
2022/10/04 11:25:15 Read took 22.623354852s

We leave users to tune as blocks can be variable size due to strings.

Can be set on the connection level with BlockBufferSize or via dsn with block_buffer_size or via specific query as shown above.