[writeboost] Add dbench test

dbench can switch the SYNC frequency in the test by its option -S and -s.

- fix: measuring range in writeback-sorting test
- DEBUGMODE is added locally to test the tests (which is false by
  default). @param :: [] is added to each Tests class
- Stack#drop_caches added to cleanup the dirties in the SSD
  and apply this to other tests

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] Sync daemon works while drop_caches

drop_caches waits for all dirty caches written back to the backing
device. If some dirties stores after this message is sent, it never
returns (it is interruptible, the kernel code is OK).
To avoid this failure, we need to work the sync daemon while
drop_caches. This periodically flushes the dirties on the RAM buffer so
this failure never happen.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] Add 4KB randwrite test

This test is to see how Writeboost is efficient for the random write.
An another test to measure the seq write of the SSD is added as the baseline.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
Merge pull request #27 from akiradeveloper/feature/add-fio-randwrite-perf-test

Feature/add fio randwrite perf test