Cache the validity mask, data pointers, and the chunk size

[taniabogatsch]

Fixes #253.

Significant performance improvement for scans.

Scan benchmark.

• CREATE TABLE tbl (col1 STRUCT(value INTEGER));
• Fill the table with 1M mock values.
• We see huge improvements both for scanning nested data, and for scanning the integer directly.

PR timings.

SELECT col1.value FROM stress2.main.table2, took 32.725833ms
SELECT col1 FROM stress2.main.table2, took 192.932708ms
SELECT * FROM stress2.main.table2, took 198.002875ms

Additionally, the included benchmark for all types improved slightly.
BenchmarkTypes-10    	       5	 248644567 ns/op

Before.

SELECT col1.value FROM stress2.main.table2, took 236.263375ms
SELECT col1 FROM stress2.main.table2, took 466.651625ms
SELECT * FROM stress2.main.table2, took 460.474834ms

BenchmarkTypes-10    	       4	 286125906 ns/op