use compression level 4 by default

[rzymek]

Work in progress

I did a comparison of file size and generation time for all possible compression levels

level	size	time
0	19360975	242
1	2761559	371
2	2209736	343
3	2467095	393
4	2158858	389
5	2287434	416
6	2281720	550
7	2299445	534
8	2293590	934
-1	2281720	534

Concusion

1. The default (-1) is level 6
2. Level 4 gives best size and very good time
3. Level 2 gives best time and very good size

Suggestion

I think it would be good to set the default to 4

[ochedru]

Thank you for your PR! Please go ahead and set a default compression level (looks like setCompressionLevel() is called only from the benchmark code in your PR).