LU-5319 mdc: manage number of modify RPCs in flight

LU-5319 mdc: manage number of modify RPCs in flight

This patch is the main client part of a new feature that supports
multiple modify metadata RPCs in parallel. Its goal is to improve
metadata operations performance of a single client, while maintening
the consistency of MDT reply reconstruction and MDT recovery
mecanisms.

It allows to manage the number of modify RPCs in flight within
the client obd structure and to assign a virtual index (the tag) to
each modify RPC to help server side cleaning of reply data.

The mdc component uses this feature to send multiple modify RPCs
in parallel.

Signed-off-by: Gregoire Pichon <gregoire.pichon@bull.net>
Change-Id: Ia707e39770e479648627611a99d0724e7070baec
Reviewed-on: http://review.whamcloud.com/14374
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>