LU-2145 server: use unified request handler for MGS

- Unify request handler. It finds target for particular request and
  calls appropriate handler for it. Generic handlers are moved to
  the unified target code. The tgt_session_info is introduced to
  store all request-related data and passed to all handlers.
- Pack reply in llog server functions early and use err_serious()
- remove obsoleted llog_origin_handle_cancel(), it is not used
  anymore
- remove push_ctxt/pop_ctxt from llog server function, it is based
  on OSD now.

Change-Id: Idded90d8112bcab74de9a58e7885fa17dd3340b8
Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Reviewed-on: http://review.whamcloud.com/4826
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
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>