LU-11421 dom: manual OST-to-DOM migration via mirroring

LU-11421 dom: manual OST-to-DOM migration via mirroring

Allow DOM mirroring, update LOV/LOD code to check not just
first component for DOM pattern but cycle through all mirrors
if any. Sanity checks allows one DOM component in a mirror
and it should be the first one. Multiple DOM components are
allowed only with the same for now.

Do OST file migration to MDT by using FLR. That can't be done
by layout swapping, because MDT data will be tied to temporary
volatile file but we want to keep data with the original file.
The mirroring allows that with the following steps:
- extent layout with new mirror on MDT, no data is copied but
  new mirror stays in 'stale' state. The reason is the same
  problem with volatile file.
- resync mirrors, now new DOM layout is filled with data.
- remove first mirror

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I1d4213196a16d6aec70861c5530910cac062e34f
Reviewed-on: https://review.whamcloud.com/35359
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>