LU-3285 test: add Data-on-MDT tests and fixes

LU-3285 test: add Data-on-MDT tests and fixes

- add test for basic DoM functionality in sanity.sh
- add test to read/write many files with small chunks of data,
  it is used for performance comparision.
- the sanity-dom.sh is added to test Data-on-MDT.
  It contains specific sanity tests and calls sanity.sh and
  sanityn.sh with limited set of test to cover the same cases
  as OSC.
- fixes for DoM code for issues revealed by tests.

Test-Parameters: mdssizegb=20 testlist=sanity-dom,dom-performance
Signed-off-by: Mikhal Pershin <mike.pershin@intel.com>
Change-Id: I17fbbf8e913cc14043703969d01ac65e6828fcc6
Reviewed-on: https://review.whamcloud.com/28020
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>