deflate: Improve level 7-9

[klauspost]

Check post-match alternative at a 2 byte offset to allow longer matches and add up to 2 literals.

Adjust minimum match cost on level 9.

Before/after pairs:

file	out	level	insize	outsize	millis	mb/s
github-june-2days-2019.json	gzkp	7	6273951764	919314231	42313	141.41
github-june-2days-2019.json	gzkp	7	6273951764	924473679	42317	141.39

github-june-2days-2019.json	gzkp	8	6273951764	904763581	53796	111.22
github-june-2days-2019.json	gzkp	8	6273951764	905294390	53747	111.32

github-june-2days-2019.json	gzkp	9	6273951764	897031618	105876	56.51
github-june-2days-2019.json	gzkp	9	6273951764	895561157	105651	56.63

nyc-taxi-data-10M.csv	gzkp	7	3325605752	741407017	46656	67.98
nyc-taxi-data-10M.csv	gzkp	7	3325605752	731632472	45047	70.40

nyc-taxi-data-10M.csv	gzkp	8	3325605752	725159779	69767	45.46
nyc-taxi-data-10M.csv	gzkp	8	3325605752	718753419	69238	45.81

nyc-taxi-data-10M.csv	gzkp	9	3325605752	709819738	130439	24.31
nyc-taxi-data-10M.csv	gzkp	9	3325605752	702234731	130544	24.29


gob-stream	gzkp	7	1911399616	298204449	12916	141.12
gob-stream	gzkp	7	1911399616	295193952	12497	145.86

gob-stream	gzkp	8	1911399616	286865198	17153	106.27
gob-stream	gzkp	8	1911399616	285374252	16652	109.46

gob-stream	gzkp	9	1911399616	277771119	41745	43.67
gob-stream	gzkp	9	1911399616	276757881	41077	44.38


silesia.tar	gzkp	7	211947520	69605688	3034	66.62
silesia.tar	gzkp	7	211947520	69331085	2934	68.88

silesia.tar	gzkp	8	211947520	68745247	4108	49.2
silesia.tar	gzkp	8	211947520	68555973	3989	50.67

silesia.tar	gzkp	9	211947520	68254323	11843	17.07
silesia.tar	gzkp	9	211947520	68158666	12278	16.46


enwik9	gzkp	7	1000000000	325327111	17413	54.77
enwik9	gzkp	7	1000000000	324323925	17014	56.05

enwik9	gzkp	8	1000000000	323311871	21067	45.27
enwik9	gzkp	8	1000000000	322268755	20626	46.24

enwik9	gzkp	9	1000000000	321637537	34016	28.04
enwik9	gzkp	9	1000000000	320812830	33955	28.09

webdevdata.org-2015-01-07-subset	TRUE	gzkp	7	53927	4014735833	746309058	34562	110.78
webdevdata.org-2015-01-07-subset	true	gzkp	7	53927	4014735833	746308934	34062	112.40

webdevdata.org-2015-01-07-subset	TRUE	gzkp	8	53927	4014735833	735523067	43270	88.48
webdevdata.org-2015-01-07-subset	true	gzkp	8	53927	4014735833	734840811	42613	89.85

webdevdata.org-2015-01-07-subset	TRUE	gzkp	9	53927	4014735833	727900632	94305	40.6
webdevdata.org-2015-01-07-subset	true	gzkp	9	53927	4014735833	726973932	95443	40.12