Add dbench and some fixes

With this branch, all tests below runs in batched way in my system. Some fixes included

```
echo git extract
dmtest run --suite writeboost -t /Backing/ -n git_extract_cache_quick
dmtest run --suite writeboost -t /Type0/ -n git_extract_cache_quick
dmtest run --suite writeboost -t /Type1/ -n git_extract_cache_quick
echo writeback sorting
dmtest run --suite writeboost -t /Backing/ -n writeback_sorting_effect
dmtest run --suite writeboost -t /Type0/ -n writeback_sorting_effect
dmtest run --suite writeboost -t /Type1/ -n writeback_sorting_effect
echo randread overhead \(HDD backed\)
dmtest run --suite writeboost -t /Backing/ -n fio_read_overhead
dmtest run --suite writeboost -t /Type0/ -n fio_read_overhead
dmtest run --suite writeboost -t /Type1/ -n fio_read_overhead
echo randread overhead \(SSD backed\)
dmtest run --suite writeboost --profile ssd -t /Backing/ -n fio_read_overhead
dmtest run --suite writeboost --profile ssd -t /Type0/ -n fio_read_overhead
dmtest run --suite writeboost --profile ssd -t /Type1/ -n fio_read_overhead
echo dbench
dmtest run --suite writeboost -t /Backing/ -n do_dbench
dmtest run --suite writeboost -t /Type0/ -n do_dbench
dmtest run --suite writeboost -t /Type1/ -n do_dbench
echo stress
dmtest run --suite writeboost -t /Backing/ -n do_stress
dmtest run --suite writeboost -t /Type0/ -n do_stress
dmtest run --suite writeboost -t /Type1/ -n do_stress
```