LU-3285 mdt: IO request handling in MDT

LU-3285 mdt: IO request handling in MDT

Add methods to handle IO requests in MDT similar to OFD.
Introduce MDS_INODEBITS_DOM bit for data on MDT, destinguish
IO requests to MDT and OST and take appropriate lock in
target code.

Change-Id: I7feaa00c381f821510ca1343b042ed5f09050ac6
Signed-off-by: Mikhal Pershin <mike.pershin@intel.com>
Reviewed-on: https://review.whamcloud.com/28013
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>