max_compaction_threshold

[Radim Kolar]

I found in stress tests that default setting this to 32 is way too high. 
Hadoop guys are using value 10 during merge sorts to not stress IO that 
much. I also discovered that filesystems like ZFS are using default io 
queue size of 10 per drive.

I tried run tests with 10, 15 and 32 and there is very little 
improvement 15 over 10, but 32 is performance drop. Test is simply 
create lot of 5m sstables by diabling minor compactions and then set it 
back and measure time elapsed.