LU-2059 llog: MGC to use OSD API for backup logs

MGC uses lvfs API to access local llogs blocking removal of old code

- MGS is converted to use OSD API for local llogs
- llog_is_empty() and llog_backup() are introduced
- initial OSD start initialize run ctxt so llog can use it too
- shrink dcache after initial scrub in osd-ldiskfs to avoid holding
  data of unlinked objects until umount.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I137e38abb0e93c12406c6610c0922f4294e67290
Reviewed-on: http://review.whamcloud.com/5049
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>